fix: Harden DATE_FORMAT and TO_TIMESTAMP DuckDB translations

[ferenc-csaky]

Handle empty strings and NULL values the same way as Flink in the mentioned DuckDB translations.

Instead of throwing an exception, the translated expression should return NULL in all of these incomprehensible situations.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 34 lines in your changes missing coverage. Please review.
✅ Project coverage is 12.99%. Comparing base (a8ef976) to head (848a3f8).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...ation/duckdb/builtin/DateFormatSqlTranslation.java	0.00%	13 Missing ⚠️
...tion/duckdb/builtin/ToTimestampSqlTranslation.java	0.00%	12 Missing ⚠️
...tion/duckdb/builtin/DuckDbSqlTranslationUtils.java	0.00%	9 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2054      +/-   ##
============================================
- Coverage     13.00%   12.99%   -0.02%     
  Complexity      803      803              
============================================
  Files           606      606              
  Lines         17437    17456      +19     
  Branches       2102     2101       -1     
============================================
  Hits           2268     2268              
- Misses        14967    14986      +19     
  Partials        202      202              

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