perf: skip building FilterRemapper when there are no parent filters

[Braedon-Wooding-Displayr]

ChildFilterDescription::from_child (and from_child_with_allowed_indices) built a FilterRemapper before checking whether parent_filters was empty. FilterRemapper::new indexes every column of the child's schema into a HashMap, and with no filters to remap that index is never used: remap_filters returns an empty description regardless of what the remapper contains.

Each plan node produces one ChildFilterDescription per child, so a union pays this cost once per child. On a 250,000-column scan a sampling profile put FilterRemapper::new at 794 of 20,373 samples, almost entirely HashMap insertion and hashing. Return ChildFilterDescription::empty() directly when parent_filters is empty, before the remapper is built.

Which issue does this PR close?

Do I need an issue for this change, it feels like a really minor fix?

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.24%. Comparing base (c7be84a) to head (c145abc).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24428      +/-   ##
==========================================
- Coverage   81.24%   81.24%   -0.01%     
==========================================
  Files        1112     1112              
  Lines      390912   390924      +12     
  Branches   390912   390924      +12     
==========================================
  Hits       317604   317604              
- Misses      54670    54678       +8     
- Partials    18638    18642       +4     

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