[fix](struct-type) fix insert null and explain for struct type

[cambyzju]

Proposed changes

Issue Number: close #xxx

Problem summary

1. support insert null for struct type;
2. fix explain const exprs for strut type got wrong result;

> explain insert into example3 values(20,{12345678,12345679}),(21,{12345678,12345679});
+------------------------------------------+
| Explain String                           |
+------------------------------------------+
| PLAN FRAGMENT 0                          |
|   OUTPUT EXPRS:                          |
|     <slot 2> 20                          |
|     <slot 3> STRUCT(12345678, 12345679)  |
|   PARTITION: UNPARTITIONED               |
|                                          |
|   OLAP TABLE SINK                        |
|     TUPLE ID: 0                          |
|     RANDOM                               |
|                                          |
|   0:VUNION                               |
|      constant exprs:                     |
|          20 | STRUCT(12345678, 12345679) |
|          21 | STRUCT(12345678, 12345679) |
+------------------------------------------+

Checklist(Required)

1. Does it affect the original behavior:
   • Yes
   • No
   • I don't know
2. Has unit tests been added:
   • Yes
   • No
   • No Need
3. Has document been added or modified:
   • Yes
   • No
   • No Need
4. Does it need to update dependencies:
   • Yes
   • No
5. Are there any changes that cannot be rolled back:
   • Yes (If Yes, please explain WHY)
   • No

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...

[github-actions]

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

[xy720]

LGTM

[github-actions]

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

[github-actions]

PR approved by anyone and no changes requested.

[github-actions]

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