Throw on undefined root fields

[user1736]

Introduces a change that makes execution throw if you try to run a query/mutation without definition.

[vladar]

There is one tricky use-case breaking with this change. Technically executor can work without custom resolvers at all - only with default resolver (e.g. with projects like Grats)

I.e. you can pass some instance as rootValue and it's methods act as resolvers. This case is handled by default resolver. See:
https://github.com/user1736/graphitation/blob/d8db64e68d5728f7de149bf009a2f66350df6ede/packages/supermassive/src/executeWithoutSchema.ts#L2348-L2350

We should account for this case and only throw if there is no "source":

Suggested change

	if (resolveFn === exeContext.fieldResolver) {
	if (resolveFn === exeContext.fieldResolver && typeof source === "undefined") {

[user1736]

fixed

[vladar]

Subscription? Alternatively we can just check path.length === 0 once

[user1736]

Subscriptions have a separate execution and this function is not called there.

As for the path, it's probably not what you think it is. For example, for new test cases I get:

{
  "prev": undefined,
  "key": "downloadFile",
  "typename": "Mutation"
}

[vladar]

Ah, right, path is an array is in graphql-js, we changed it to linked list for perf I guess. In our case it's just !path.prev. But whatever, this works too.