Align GraphQLErrors and NetworkError errors. Fix docs

[korywka]

1. Continue to align error types according to changes in Align NetworkError and GraphQLErrors types #8424

2. Try to fix Typescript error when ignoring errors #8896

const errorLink = onError(({ response, operation }) => {
  if (operation.operationName === "IgnoreErrorsQuery") {
    response.errors = null;
  }
});

The error is:

Type 'null' is not assignable to type 'readonly GraphQLError[] | undefined'

If we look at ExecutionResult types at graphql, errors can't be null:

https://github.com/graphql/graphql-js/blob/e289e6412ca59cd6298796d61efa19d78eeb945c/src/execution/execute.ts#L127-L134

So I suggest to fix docs to assign undefined instead of null.

Also we need to check if response exists, cause it is optional:

apollo-client/src/link/error/index.ts

Lines 9 to 15 in cbcf951

	export interface ErrorResponse {
	graphQLErrors?: ReadonlyArray<GraphQLError>;
	networkError?: Error | ServerError | ServerParseError;
	response?: ExecutionResult;
	operation: Operation;
	forward: NextLink;
	}

So my suggestion is to check like this:

if (response && operation.operationName === "IgnoreErrorsQuery") {
  response.errors = undefined;
}

The bad part is that a lot of tests are broken in this rc:

Test Suites: 93 failed, 18 passed, 111 total

I could not brake them 🤷‍♂️. So please, verify these changes and merge them on your own risk. Thanks!

[korywka]

@StephenBarlow @brainkim Please let me know if you are interested in this PR and I will resolve conflicts.