Fix file header insertion before a blank line

[sharwell]

Fixes #2415

[codecov]

Codecov Report

Merging #2421 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2421      +/-   ##
==========================================
+ Coverage   96.43%   96.44%   +<.01%     
==========================================
  Files         714      715       +1     
  Lines       92522    92616      +94     
  Branches     3444     3447       +3     
==========================================
+ Hits        89226    89320      +94     
  Misses       2403     2403              
  Partials      893      893

Impacted Files	Coverage Δ
...es/DocumentationRules/FileHeaderCodeFixProvider.cs	96.96% <100%> (+0.16%)	⬆️
...est/DocumentationRules/NoXmlFileHeaderUnitTests.cs	100% <100%> (ø)	⬆️
...yzers/StyleCop.Analyzers.Test/WorkItemAttribute.cs	100% <100%> (ø)
...nalyzers/StyleCop.Analyzers.Test/AttributeTests.cs	100% <100%> (ø)	⬆️

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