Don't use string 'n/a' where the rest of the code expects a numeric value

[dvdoug]

https://github.com/sebastianbergmann/php-code-coverage/blob/master/src/Report/Html/Renderer.php#L80 expects a numeric value

Fixes #781

[codecov]

Codecov Report

Merging #782 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #782   +/-   ##
=========================================
  Coverage     84.29%   84.29%           
  Complexity     1074     1074           
=========================================
  Files            56       56           
  Lines          3234     3234           
=========================================
  Hits           2726     2726           
  Misses          508      508           

Impacted Files	Coverage Δ	Complexity Δ
src/Report/Html/Renderer/File.php	92.09% <100.00%> (ø)	184.00 <0.00> (ø)

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