feat: tests for nan and nulls in percentile and median UDAF

[theirix]

Which issue does this PR close?

• Closes Review NaN handling in median and approx_median #3039.

Rationale for this change

I recently refined null support for median / percentile_cont and their approximate counterparts approx_median, appprox_percentile_cont`

With the following tests for NaN for percentiles, the behaviour of all four functions for NaN and nulls is defined and covered by tests in aggregate.slt

What changes are included in this PR?

Missing tests for NaN for two functions

Are these changes tested?

Verified with DuckDB and PostgreSQL

Are there any user-facing changes?

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.37%. Comparing base (32badcb) to head (6c36e88).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #24580    +/-   ##
========================================
  Coverage   81.37%   81.37%            
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  Files        1116     1116            
  Lines      397509   397661   +152     
  Branches   397509   397661   +152     
========================================
+ Hits       323461   323591   +130     
- Misses      55110    55126    +16     
- Partials    18938    18944     +6     

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