[CHORE] Enable sources to return empty tables

[colin-ho]

Sources in the native executor should still return empty tables if they didn't read anything.

This unlocks a bunch of tests.

[codspeed-hq]

CodSpeed Performance Report

Merging #2915 will not alter performance

_{Comparing colin/swordfish-empty-dfs (9aaac3f) with main (a9fdd19)}

Summary

✅ 17 untouched benchmarks

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 78.38%. Comparing base (a9fdd19) to head (9aaac3f).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/daft-local-execution/src/sources/in_memory.rs	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2915      +/-   ##
==========================================
+ Coverage   77.57%   78.38%   +0.81%     
==========================================
  Files         596      596              
  Lines       70687    69685    -1002     
==========================================
- Hits        54835    54626     -209     
+ Misses      15852    15059     -793     

Flag	Coverage Δ
	78.38% <83.33%> (+0.81%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/daft-local-execution/src/pipeline.rs	91.32% <100.00%> (+0.07%)	⬆️
src/daft-local-execution/src/sources/scan_task.rs	91.53% <ø> (ø)
src/daft-local-execution/src/sources/in_memory.rs	90.90% <66.66%> (-9.10%)	⬇️

... and 20 files with indirect coverage changes