fix: handle null date and timestamp format arguments

[lyne7-sc]

Which issue does this PR close?

• Closes to_date, to_timestamp, and to_unixtime fail on NULL string and format arguments #23640.

Rationale for this change

The formatted to_date and to_timestamp* functions can produce internal errors when the input string is NULL or when all format arguments are NULL.

A NULL value in a format column is also treated as an empty string instead of being skipped.

to_unixtime uses the same timestamp parsing path, so it is affected as well.

What changes are included in this PR?

• Return NULL when the scalar input string is NULL.
• Return NULL when all format arguments are NULL.
• Skip NULL formats in array rows.
• Keep returning a parsing error when all non-NULL formats fail.

Are these changes tested?

Yes, SLTs were added for to_date, to_timestamp, and to_unixtime.

Are there any user-facing changes?

Yes, only the bug fix described above. No API changes.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 73.33333% with 4 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@3e058f0). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
datafusion/functions/src/datetime/common.rs	73.33%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #23641   +/-   ##
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  Coverage        ?   80.65%           
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  Files           ?     1086           
  Lines           ?   366105           
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  Hits            ?   295279           
  Misses          ?    53230           
  Partials        ?    17596           

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