refactor: null checkers rework

[FrediKats]

Some preparation for #88

[codecov]

Codecov Report

Merging #101 into master will not change coverage.
The diff coverage is 71.42%.

@@           Coverage Diff           @@
##           master     #101   +/-   ##
=======================================
  Coverage   33.01%   33.01%           
=======================================
  Files          13       13           
  Lines         312      312           
  Branches       36       36           
=======================================
  Hits          103      103           
  Misses        200      200           
  Partials        9        9           

Impacted Files	Coverage Δ
WopiHost.Discovery/HttpDiscoveryFileProvider.cs	0.00% <0.00%> (ø)
WopiHost.FileSystemProvider/WopiSecurityHandler.cs	0.00% <0.00%> (ø)
WopiHost.Discovery/WopiDiscoverer.cs	86.44% <100.00%> (ø)
WopiHost.Url/CollectionExtensions.cs	100.00% <100.00%> (ø)
WopiHost.Url/WopiUrlSettings.cs	37.83% <100.00%> (ø)

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[petrsvihlik]

hi, thanks for the PR! I'll take a look at it tonight ;)