fix: map.get on empty dataset

[universalmind303]

closes #3891

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.72%. Comparing base (567ae9a) to head (c73a38f).
Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3892      +/-   ##
==========================================
- Coverage   78.46%   76.72%   -1.74%     
==========================================
  Files         767      769       +2     
  Lines       97108   100130    +3022     
==========================================
+ Hits        76193    76829     +636     
- Misses      20915    23301    +2386     

Files with missing lines	Coverage Δ
src/daft-core/src/array/ops/map.rs	87.87% <100.00%> (+0.37%)	⬆️

... and 40 files with indirect coverage changes

[universalmind303]

pretty sure the llvm-cov is wrong here.

The only file changed as increased coverage, but the overall coverage went down?