Ensure Date(Time)Picker does not clear on partial entry

[philippjfr]

The DatePicker/DatetimePicker updateModelValue and handleChange treated "field is fully empty" (null) and "field is mid-edit with an incomplete section" (a non-null but structurally invalid dayjs object) identically. MUI fires onAccept on every keystroke for field-only edits (not just on blur/Enter), so typing a single digit into the year section immediately produced an invalid intermediate dayjs value, which the old code treated as "clear" whenever clearable=True — wiping the field and committing None to Python.

This ensures an invalid partial date entry no longer triggers a clear and commit of None.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.67%. Comparing base (0a6ef7a) to head (09b83b6).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #720      +/-   ##
==========================================
- Coverage   84.23%   83.67%   -0.56%     
==========================================
  Files          34       34              
  Lines        3958     3958              
==========================================
- Hits         3334     3312      -22     
- Misses        624      646      +22     

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