Resolve nullable reference warnings

[waf]

Just cleaning up a few nullable reference warnings that recently crept in.

[codecov[bot]]

Codecov Report

Merging #275 (35d2151) into main (a9f36f7) will decrease coverage by 0.2%. The diff coverage is 50.0%.

@@           Coverage Diff           @@
##            main    #275     +/-   ##
=======================================
- Coverage   77.5%   77.4%   -0.2%     
=======================================
  Files         81      81             
  Lines       5475    5475             
  Branches     718     720      +2     
=======================================
- Hits        4248    4238     -10     
- Misses       965     972      +7     
- Partials     262     265      +3     

Impacted Files	Coverage Δ
CSharpRepl.Services/Disassembly/Disassembler.cs	91.8% <0.0%> (-1.1%)	:arrow_down:
CSharpRepl.Services/Roslyn/RoslynServices.cs	83.8% <0.0%> (-0.4%)	:arrow_down:
...ervices/Completion/OpenAI/OpenAICompleteService.cs	75.9% <100.0%> (ø)
CSharpRepl.Services/Configuration.cs	81.2% <100.0%> (-4.5%)	:arrow_down:

... and 1 file with indirect coverage changes