Add a test for empty field argument list

... within parens.

tests/Language/GraphQL/AST/ParserSpec.hs

@@ -75,7 +75,10 @@ spec = describe "Parser" $ do
                mutation auth{
                  test(username: """username""", password: """password""")
                }|]
- 
+
+       it "fails to parse an empty argument list in parens" $
+           parse document "" `shouldFailOn` "{ test() }"
+
        it "accepts any arguments" $ mapSize (const 10) $ property $ \xs ->
             let
                 query' :: Text