[FEAT] fill_null expression

[colin-ho]

Closes #1904

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.28%. Comparing base (2e644b9) to head (3d68dcc).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2089      +/-   ##
==========================================
+ Coverage   85.24%   85.28%   +0.04%     
==========================================
  Files          68       68              
  Lines        7258     7267       +9     
==========================================
+ Hits         6187     6198      +11     
+ Misses       1071     1069       -2     

Files	Coverage Δ
daft/expressions/expressions.py	91.94% <100.00%> (+0.06%)	⬆️
daft/series.py	92.95% <100.00%> (+0.07%)	⬆️

... and 1 file with indirect coverage changes

[samster25]

@kevinzwang You have time Wednesday to take a look?

[kevinzwang]

I'm not familiar enough with our usage of semantic id to fully understand that part, but otherwise LGTM!