update docstring about QgsDateTimeEdit with NULL values

[Gustry]

Description

If you have a QgsDateTimeEdit, you mustn't call date() or time() on it as it can't handle NULL values. Only dateTime().

This PR needs to be backported for documentation in the API.

In a following PR, I can add date() and time() to return an empty QDateTime if needed. But I'm not sure if it can be backported.
If it is, then this PR can be updated.

It's not related to #31849

Checklist

• Commit messages are descriptive and explain the rationale for changes
• Commits which fix bugs include Fixes #11111 at the bottom of the commit message
• I have read the QGIS Coding Standards and this PR complies with them
• New unit tests have been added for core changes
• I have run the scripts/prepare-commit.sh script before each commit
• I have evaluated whether it is appropriate for this PR to be backported, backport requests are left as label or comment