Replace null assertions with checkNotNull

[fractalwrench]

Goal

Replaces some existing null assertions with checkNotNull(), which is a bit more explicit IMO that a NPE is not expected.

Testing

Relied on existing test coverage.

[codecov]

Codecov Report

Merging #138 (0dd59b1) into master (1258907) will increase coverage by 0.01%.
The diff coverage is 37.50%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #138      +/-   ##
==========================================
+ Coverage   76.12%   76.14%   +0.01%     
==========================================
  Files         318      318              
  Lines       10117    10121       +4     
  Branches     1484     1486       +2     
==========================================
+ Hits         7702     7707       +5     
+ Misses       1812     1809       -3     
- Partials      603      605       +2     

Files	Coverage Δ
...mbracesdk/capture/webview/EmbraceWebViewService.kt	79.16% <0.00%> (-2.09%)	⬇️
...racesdk/capture/crumbs/EmbraceBreadcrumbService.kt	74.70% <33.33%> (-0.20%)	⬇️
...roid/embracesdk/ndk/EmbraceNdkServiceRepository.kt	71.69% <50.00%> (+1.10%)	⬆️

... and 1 file with indirect coverage changes