Remove unnecessary invariant in favor of explicit cast

See open TS issue microsoft/TypeScript#31613

Marked with FIXME for when TS feature/bug fix lands upstream

[This code will likely be removed in v17 prior to the feature landing, but may be retained within v16 for longer.]

Similar code "works" at https://github.com/graphql/graphql-js/blob/1f8ba95c662118452bd969c6b26ba4e9050c55da/src/error/GraphQLError.ts#L47 because all the option properties are optional.

Author: yaacovCR

src/execution/subscribe.ts

@@ -1,5 +1,4 @@
 import { inspect } from '../jsutils/inspect';
-import { invariant } from '../jsutils/invariant';
 import { isAsyncIterable } from '../jsutils/isAsyncIterable';
 import { isPromise } from '../jsutils/isPromise';
 import type { Maybe } from '../jsutils/Maybe';
@@ -108,11 +107,10 @@ function toNormalizedArgs(args: BackwardsCompatibleArgs): ExecutionArgs {
     return firstArg;
   }
 
-  invariant(args[1] != null, 'Must provide document.');
-
   return {
     schema: firstArg,
-    document: args[1],
+    // FIXME: when underlying TS bug fixed, see https://github.com/microsoft/TypeScript/issues/31613
+    document: args[1] as DocumentNode,
     rootValue: args[2],
     contextValue: args[3],
     variableValues: args[4],