update test with null values

- addresses graphql-python#118
- initial implementation by @yen223 in PR graphql-python#119

Author: jaemk

graphql/language/tests/fixtures.py

@@ -53,7 +53,7 @@
 }
 
 {
-  unnamed(truthy: true, falsey: false),
+  unnamed(truthy: true, falsey: false, nullish: null),
   query
 }
 """

graphql/language/tests/test_parser.py

@@ -90,11 +90,8 @@ def test_does_not_accept_fragments_spread_of_on():
     assert 'Syntax Error GraphQL (1:9) Expected Name, found }' in excinfo.value.message
 
 
-def test_does_not_allow_null_value():
-    with raises(GraphQLSyntaxError) as excinfo:
-        parse('{ fieldWithNullableStringInput(input: null) }')
-
-    assert 'Syntax Error GraphQL (1:39) Unexpected Name "null"' in excinfo.value.message
+def test_allows_null_value():
+    parse('{ fieldWithNullableStringInput(input: null) }')
 
 
 def test_parses_multi_byte_characters():
@@ -130,7 +127,8 @@ def tesst_allows_non_keywords_anywhere_a_name_is_allowed():
         'mutation',
         'subscription',
         'true',
-        'false'
+        'false',
+        'null',
     ]
 
     query_template = '''

graphql/language/tests/test_printer.py

@@ -118,7 +118,7 @@ def test_prints_kitchen_sink():
 }
 
 {
-  unnamed(truthy: true, falsey: false)
+  unnamed(truthy: true, falsey: false, nullish: null)
   query
 }
 '''

graphql/language/tests/test_visitor.py

@@ -562,6 +562,12 @@ def leave(self, node, key, parent, *args):
         ['enter', 'BooleanValue', 'value', 'Argument'],
         ['leave', 'BooleanValue', 'value', 'Argument'],
         ['leave', 'Argument', 1, None],
+        ['enter', 'Argument', 2, None],
+        ['enter', 'Name', 'name', 'Argument'],
+        ['leave', 'Name', 'name', 'Argument'],
+        ['enter', 'NullValue', 'value', 'Argument'],
+        ['leave', 'NullValue', 'value', 'Argument'],
+        ['leave', 'Argument', 2, None],
         ['leave', 'Field', 0, None],
         ['enter', 'Field', 1, None],
         ['enter', 'Name', 'name', 'Field'],