[Fix](function) fix pad functions behaviour of empty pad string

[zclllyybb]

Proposed changes

Issue Number: close #xxx

before:

mysql> SELECT lpad("", 0, "");
+-----------------+
| lpad('', 0, '') |
+-----------------+
| NULL            |
+-----------------+
1 row in set (0.10 sec)

after:

mysql> SELECT lpad("", 0, "");
+-----------------+
| lpad('', 0, '') |
+-----------------+
|                 |
+-----------------+
1 row in set (0.10 sec)

doc pr: apache/doris-website#647

Further comments

If this is a relatively large or complex change, kick off the discussion at dev@doris.apache.org by explaining why you chose the solution you did and what alternatives you considered, etc...

[doris-robot]

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[zclllyybb]

run buildall

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[github-actions]

PR approved by anyone and no changes requested.

[doris-robot]

TeamCity be ut coverage result:
Function Coverage: 35.67% (8988/25198)
Line Coverage: 27.32% (74285/271918)
Region Coverage: 26.56% (38395/144585)
Branch Coverage: 23.37% (19580/83780)
Coverage Report: http://coverage.selectdb-in.cc/coverage/cc85cb4ede8da523b6db9a96d474820b83b862f6_cc85cb4ede8da523b6db9a96d474820b83b862f6/report/index.html

[doris-robot]

TPC-H: Total hot run time: 41745 ms

machine: 'aliyun_ecs.c7a.8xlarge_32C64G'
scripts: https://github.com/apache/doris/tree/master/tools/tpch-tools
Tpch sf100 test result on commit cc85cb4ede8da523b6db9a96d474820b83b862f6, data reload: false

------ Round 1 ----------------------------------
q1	17613	4291	4266	4266
q2	2021	189	190	189
q3	10477	1183	1251	1183
q4	10211	845	811	811
q5	7482	2694	2814	2694
q6	219	134	134	134
q7	1018	593	595	593
q8	9340	2140	2082	2082
q9	9303	6727	6759	6727
q10	9280	3855	3865	3855
q11	441	240	244	240
q12	449	231	236	231
q13	18634	3217	3016	3016
q14	246	228	246	228
q15	517	480	475	475
q16	509	406	400	400
q17	976	721	732	721
q18	8384	7712	7697	7697
q19	4200	1530	1516	1516
q20	673	319	330	319
q21	5179	4090	4134	4090
q22	342	278	282	278
Total cold run time: 117514 ms
Total hot run time: 41745 ms

----- Round 2, with runtime_filter_mode=off -----
q1	4509	4385	4426	4385
q2	369	264	258	258
q3	3105	2979	2735	2735
q4	1837	1553	1706	1553
q5	5537	5491	5483	5483
q6	208	121	124	121
q7	2352	1959	1977	1959
q8	3212	3394	3380	3380
q9	8569	8696	8624	8624
q10	3924	3833	3786	3786
q11	580	507	501	501
q12	807	612	601	601
q13	16517	3103	3150	3103
q14	292	277	269	269
q15	518	482	491	482
q16	469	407	411	407
q17	1738	1481	1461	1461
q18	7722	7695	7481	7481
q19	1635	1586	1586	1586
q20	1939	1782	1765	1765
q21	5042	4756	4767	4756
q22	559	484	477	477
Total cold run time: 71440 ms
Total hot run time: 55173 ms

[doris-robot]

TPC-DS: Total hot run time: 187697 ms

machine: 'aliyun_ecs.c7a.8xlarge_32C64G'
scripts: https://github.com/apache/doris/tree/master/tools/tpcds-tools
TPC-DS sf100 test result on commit cc85cb4ede8da523b6db9a96d474820b83b862f6, data reload: false

query1	905	366	350	350
query2	6415	2391	2382	2382
query3	6642	202	208	202
query4	22791	21357	21219	21219
query5	4215	418	423	418
query6	256	194	166	166
query7	4581	293	285	285
query8	241	188	186	186
query9	8583	2388	2381	2381
query10	438	243	251	243
query11	14783	14171	14082	14082
query12	133	89	91	89
query13	1653	363	362	362
query14	9675	8392	8365	8365
query15	265	161	167	161
query16	8059	267	262	262
query17	1761	569	533	533
query18	2100	279	262	262
query19	205	153	143	143
query20	93	86	84	84
query21	199	134	130	130
query22	5054	4846	4840	4840
query23	34222	33435	33542	33435
query24	6738	2955	2945	2945
query25	526	387	355	355
query26	698	156	151	151
query27	1943	318	330	318
query28	3755	2068	2040	2040
query29	857	626	599	599
query30	218	155	153	153
query31	976	779	709	709
query32	93	50	70	50
query33	470	243	245	243
query34	871	470	474	470
query35	764	682	678	678
query36	1006	918	898	898
query37	105	66	64	64
query38	2850	2763	2751	2751
query39	1614	1553	1565	1553
query40	198	121	121	121
query41	40	38	38	38
query42	104	101	95	95
query43	592	575	590	575
query44	1062	722	734	722
query45	265	247	254	247
query46	1069	690	715	690
query47	1994	1902	1876	1876
query48	375	298	294	294
query49	767	396	395	395
query50	768	379	373	373
query51	6897	6843	6734	6734
query52	101	86	90	86
query53	351	287	289	287
query54	512	422	417	417
query55	74	71	71	71
query56	236	216	214	214
query57	1240	1160	1194	1160
query58	212	192	196	192
query59	3559	3300	3363	3300
query60	251	224	234	224
query61	90	83	87	83
query62	585	476	489	476
query63	301	284	277	277
query64	8387	7412	7361	7361
query65	3114	3086	3100	3086
query66	786	341	339	339
query67	15308	15074	15126	15074
query68	4565	532	531	531
query69	481	306	291	291
query70	1186	1142	1173	1142
query71	389	266	258	258
query72	7377	2527	2378	2378
query73	706	321	323	321
query74	6612	6202	6144	6144
query75	3326	2636	2579	2579
query76	2265	888	935	888
query77	414	262	259	259
query78	10710	10082	10056	10056
query79	2443	506	514	506
query80	1028	429	426	426
query81	515	218	216	216
query82	882	91	92	91
query83	266	164	161	161
query84	249	91	86	86
query85	1033	265	260	260
query86	452	284	329	284
query87	3304	3139	3159	3139
query88	4224	2322	2312	2312
query89	473	383	385	383
query90	2075	183	182	182
query91	121	98	98	98
query92	66	46	46	46
query93	1831	505	494	494
query94	1158	185	182	182
query95	389	291	301	291
query96	593	270	267	267
query97	3169	2963	2959	2959
query98	231	216	211	211
query99	1178	912	892	892
Total cold run time: 269702 ms
Total hot run time: 187697 ms

[HappenLee]

LGTM

[github-actions]

PR approved by at least one committer and no changes requested.