fix NoMethodError

NoMethodError - undefined method `gsub' for nil:NilClass:
        barkeep/lib/syntax_highlighter.rb:51:in `global_highlighting'
        barkeep/lib/syntax_highlighter.rb:25:in `colorize_blob'
        barkeep/lib/git_diff_utils.rb:48:in `block in get_tagged_commit_diffs'
        barkeep/lib/git_diff_utils.rb:26:in `map'
        barkeep/lib/git_diff_utils.rb:26:in `get_tagged_commit_diffs'
        barkeep/barkeep_server.rb:327:in `block in <class:BarkeepServer>'

Author: DrVanScott

lib/syntax_highlighter.rb

@@ -48,7 +48,9 @@ def self.pygmentize(file_type, text)
   # NOTE(caleb): It might be possible/better to do this in Python. However, that will probably involve
   # modifying Pygments (monkey-patching isn't so simple in Python) and in general will be more work.
   def self.global_highlighting(pygmentized_text)
-    pygmentized_text.gsub(/[ \t]+$/) { |whitespace| "<span class='trailingWhitespace'>#{whitespace}</span>" }
+    unless pygmentized_text.nil?
+      pygmentized_text.gsub(/[ \t]+$/) { |whitespace| "<span class='trailingWhitespace'>#{whitespace}</span>" }
+    end
   end
 
   def self.redis_cache_key(repo_name, blob)