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

be/src/vec/functions/function_string.h

@@ -1711,9 +1711,9 @@ class FunctionStringPad : public IFunction {
                                                 res_chars, res_offsets);
                     continue;
                 }
+
+                // make compatible with mysql. return empty string if pad is empty
                 if (pad_char_size == 0) {
-                    // return NULL when the string to be paded is missing
-                    null_map_data[i] = true;
                     StringOP::push_empty_string(i, res_chars, res_offsets);
                     continue;
                 }

regression-test/data/correctness_p0/test_string_pad_function.out

@@ -7,3 +7,33 @@
 \N
 10:00
 
+-- !rpad1 --
+
+
+-- !rpad2 --
+
+
+-- !rpad3 --
+\N
+
+-- !rpad4 --
+\N
+
+-- !rpad5 --
+\N
+
+-- !lpad1 --
+
+
+-- !lpad2 --
+
+
+-- !lpad3 --
+\N
+
+-- !lpad4 --
+\N
+
+-- !lpad5 --
+\N
+

regression-test/suites/correctness_p0/test_string_pad_function.groovy

@@ -43,4 +43,15 @@ suite("test_string_pad_function") {
     qt_select_rpad """
         select CASE WHEN table_pad.a = 1 THEN CONCAT(RPAD(b, 2, 0), ':00') END result from table_pad order by result;
     """
+
+    qt_rpad1 """ SELECT rpad("", 5, ""); """
+    qt_rpad2 """ SELECT rpad("123", 5, ""); """
+    qt_rpad3 """ SELECT rpad("123", -1, ""); """
+    qt_rpad4 """ SELECT rpad(NULL, 1, ""); """
+    qt_rpad5 """ SELECT rpad("123", 0, NULL); """
+    qt_lpad1 """ SELECT lpad("", 5, ""); """
+    qt_lpad2 """ SELECT lpad("123", 5, ""); """
+    qt_lpad3 """ SELECT lpad("123", -1, ""); """
+    qt_lpad4 """ SELECT lpad(NULL, 0, ""); """
+    qt_lpad5 """ SELECT lpad("123", 2, NULL); """
 }