Update NotNullOrEmptyMatcher to also implement IStringMatcher

[StefH]

No description provided.

[codecov]

Codecov Report

Merging #654 (04b5bd6) into master (57cc616) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #654      +/-   ##
==========================================
+ Coverage   74.65%   74.68%   +0.02%     
==========================================
  Files         141      141              
  Lines        5934     5941       +7     
  Branches      635      635              
==========================================
+ Hits         4430     4437       +7     
  Misses       1302     1302              
  Partials      202      202              

Impacted Files	Coverage Δ
src/WireMock.Net/Matchers/NotNullOrEmptyMatcher.cs	91.66% <100.00%> (+3.43%)	⬆️

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