Add failing test for empty templates

Author: fschwahn

test/engine_test.rb

@@ -189,5 +189,10 @@ def test_inline_source_maps
       assert_match /sourceMappingURL/, output
       assert_match /.foo/, output
     end
+
+    def test_empty_template
+      output = Engine.new('').render
+      assert_equal '', output
+    end
   end
 end