update ast_from_value/value_from_ast utils

- add None ast conversion test

Author: jaemk

graphql/utils/ast_from_value.py

@@ -15,7 +15,7 @@ def ast_from_value(value, type=None):
         return ast_from_value(value, type.of_type)
 
     if value is None:
-        return None
+        return ast.NullValue()
 
     if isinstance(value, list):
         item_type = type.of_type if isinstance(type, GraphQLList) else None

graphql/utils/tests/test_ast_from_value.py

@@ -37,6 +37,10 @@ def test_it_converts_string_values_to_asts():
     assert ast_from_value('123') == ast.StringValue('123')
 
 
+def test_it_converts_null_values_to_asts():
+    assert ast_from_value(None) == ast.NullValue()
+
+
 my_enum = GraphQLEnumType(
     'MyEnum', {
         'HELLO': GraphQLEnumValue(1),

graphql/utils/value_from_ast.py

@@ -14,6 +14,9 @@ def value_from_ast(value_ast, type, variables=None):
     if value_ast is None:
         return None
 
+    if isinstance(value_ast, ast.NullValue):
+        return None
+
     if isinstance(value_ast, ast.Variable):
         variable_name = value_ast.name.value
         if not variables or variable_name not in variables: