[Enhancement] to_json return null when unsupported

[Seaven]

Why I'm doing:

What I'm doing:

origin:

MySQL td> select to_json(map{null:null});
(1064, 'key of Map should not be null')

update:

MySQL td> select /*+SET_VAR(sql_mode='ONLY_FULL_GROUP_BY,ALLOW_THROW_EXCEPTION')*/ to_json(map{null:null});
(1064, 'key of Map should not be null')
MySQL td> select /*+SET_VAR(sql_mode='ONLY_FULL_GROUP_BY')*/ to_json(map{null:null});
+-------------------------+
| to_json(map{NULL:NULL}) |
+-------------------------+
| <null>                  |
+-------------------------+
0 rows in set

Fixes #issue

What type of PR is this:

• BugFix
• Feature
• Enhancement
• Refactor
• UT
• Doc
• Tool

Does this PR entail a change in behavior?

• Yes, this PR will result in a change in behavior.
• No, this PR will not result in a change in behavior.

If yes, please specify the type of change:

• Interface/UI changes: syntax, type conversion, expression evaluation, display information
• Parameter changes: default values, similar parameters but with different default values
• Policy changes: use new policy to replace old one, functionality automatically enabled
• Feature removed
• Miscellaneous: upgrade & downgrade compatibility, etc.

Checklist:

• I have added test cases for my bug fix or my new feature
• This pr needs user documentation (for new or modified features or behaviors)
  • I have added documentation for my new feature or new function
• This is a backport pr

Bugfix cherry-pick branch check:

• I have checked the version labels which the pr will be auto-backported to the target branch
  • 3.3
  • 3.2
  • 3.1
  • 3.0
  • 2.5

[github-actions]

[FE Incremental Coverage Report]

✅ pass : 0 / 0 (0%)

[github-actions]

[BE Incremental Coverage Report]

✅ pass : 21 / 25 (84.00%)

file detail

	path	covered_line	new_line	coverage	not_covered_line_detail
🔵	be/src/exprs/cast_expr_json.cpp	18	22	81.82%	[225, 226, 230, 231]
🔵	be/src/exprs/function_context.cpp	2	2	100.00%	[]
🔵	be/src/exprs/json_functions.cpp	1	1	100.00%	[]