astFromValue fails with a custom scalar serializing to an object value

When you have a scalar with a `serialize` function that returns an object, it is not possible to convert it to AST.
So it doesn't allow you to print a schema that has an input value with an object as a default value.

Reproduction -> https://github.com/graphql/graphql-js/pull/4086
Proposed fix -> https://github.com/graphql/graphql-js/pull/4087

```ts
const JSONScalar = new GraphQLScalarType({
name: "JSON",
serialize(value) {
return value;
},
});

const schema = new GraphQLSchema({
types: [JSONScalar],
query: new GraphQLObjectType({
name: "Query",
fields: {
test: {
type: GraphQLString,
args: {
i: {
type: JSONScalar,
defaultValue: { object: true },
},
},
},
},
}),
});
```