starrocks [Enhancement] Change the parameter strict_mode to null_if_not_found of dict

The last optional parameter for dict_mapping is strict_mode(boolean), and its behavior if following:

If strict_mode is True : return an exception if the key is not found in dictionary table. If strict_mode is False(Default) : return null if the key is not found in dictionary table.

We change the strict_mode into null_if_not_found to make more clear for user to use this function, and the behavior of null_if_not_found is following: If null_if_not_found is True : return null if the key is not found in dictionary table. If null_if_not_found is False(Default) : return an exception if the key is not found in dictionary table.

Fixes #40897

What type of PR is this:

[ ] BugFix
[ ] Feature
[x] Enhancement
[ ] Refactor
[ ] UT
[ ] Doc
[ ] Tool

Does this PR entail a change in behavior?

[x] 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
[x] 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:

[x] 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