fix(datepicker): null model with timezone throws exception

[Splaktar]

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• The commit message follows our guidelines
• Tests for the changes have been added or this is not a bug fix / enhancement
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PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Enhancement
[ ] Documentation content changes
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

• we don't verify that the model value is not null or undefined before accessing
  .getTimezoneOffset() and this causes an exception to be thrown in those cases

Issue Number:
Fixes #12025

What is the new behavior?

• verify that the model value is not null or undefined before accessing
  .getTimezoneOffset()

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information