Add null check to getAllImplementsInterfaceNodes()

[saerdnaer]

With some faulty schemas or responses iface in the following section can be null:

graphql-js/src/type/validate.js

Lines 596 to 603 in 6609d39

	function getAllImplementsInterfaceNodes(
	type: GraphQLObjectType,
	iface: GraphQLInterfaceType,
	): $ReadOnlyArray<NamedTypeNode> {
	return getAllSubNodes(type, typeNode => typeNode.interfaces).filter(
	ifaceNode => ifaceNode.name.value === iface.name,
	);
	}

If fails then with [error] Cannot read property 'name' of null. Adding a null check yields the much more understandable error message
Type $typename must only implement Interface types, it cannot implement null.

[IvanGoncharov]

@saerdnaer Thanks for reporting 👍
It's definitely a bug 🐞

[IvanGoncharov]

@saerdnaer Looking at the sources I think we make a mistake allowing invalid values to propagate to schema validate they should be rejected in GraphQLObjectType constructor.
As I understand this is not a blocker, right?
At the moment, we trying to freeze our codebase since we are working on converting the entire project to TypeScript so can we postpone this issue?

[ashutosh1919]

@IvanGoncharov and @saerdnaer , As I understand from the issue that we need to add check for iface and we need to raise error. How can we raise error? And how can I pass description. Can you plz guide me? I want to raise PR for this issue.