fix(cubesql): Allow `NULL` values in `CASE`

[MazterQyou]

Check List

• [x] Tests has been run in packages where changes made if available
• [x] Linter has been run for changed code
• [x] Tests for the changes have been added if not covered yet
• [ ] Docs have been added / updated if required

Description of Changes Made

This PR fixes an issue introduced with 2b7cc30 preventing evaluation of NULL values in CASE expression in some cases.

[codecov[bot]]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (a96831d) 47.95% compared to head (eb32fa0) 67.90%. Report is 3 commits behind head on master.

Files	Patch %	Lines
rust/cubesql/cubesql/src/compile/engine/df/scan.rs	0.00%	4 Missing :warning:

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #7761       +/-   ##
===========================================
+ Coverage   47.95%   67.90%   +19.94%     
===========================================
  Files         154      345      +191     
  Lines       20920    58074    +37154     
  Branches     5379     5379               
===========================================
+ Hits        10032    39433    +29401     
- Misses      10146    17899     +7753     
  Partials      742      742               

Flag	Coverage Δ
cube-backend	47.97% <ø> (+0.02%)	:arrow_up:
cubesql	79.11% <0.00%> (?)

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