Do not add additional newline to unified diff when completely empty

[znbailey]

Fixes #98

[codecov]

Codecov Report

Merging #99 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master      #99   +/-   ##
=========================================
  Coverage     99.11%   99.11%           
- Complexity      211      212    +1     
=========================================
  Files            12       12           
  Lines           563      563           
=========================================
  Hits            558      558           
  Misses            5        5           

Impacted Files	Coverage Δ	Complexity Δ
src/Output/UnifiedDiffOutputBuilder.php	99.07% <100.00%> (ø)	37.00 <0.00> (+1.00)

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