Return an empty Array when there aren't lines to parse.

lib/gitlab/diff/parser.rb

@@ -4,6 +4,8 @@ module Gitlab
       include Enumerable
       def parse(lines)
+        return [] if lines.blank?
+
         @lines = lines
         line_obj_index = 0
         line_old = 1

spec/lib/gitlab/diff/parser_spec.rb

@@ -90,4 +90,9 @@ eos
       end
     end
   end
+
+  context 'when lines is empty' do
+    it { expect(parser.parse([])).to eq([]) }
+    it { expect(parser.parse(nil)).to eq([]) }
+  end
 end