[Auth] nil out SafariViewController when presentation finishes

[ncooke3]

I figured how to easily reproduce #13645:

1. Launch app on iOS simulator
2. Navigate to the phone number view controller and enter a phone #, tap Send Verification Code
3. The reCaptcha flow should start. Dismiss the presented controller by tapping Done in top left or swiping down
4. An error should be propagated to indicate the user cancelled action. On Firebase 10.29.0, the error will be caught and displayed. On 11.2.0, nothing happens.

This PR fixes the issue and the error propagates.

Compared to v10 implementation, the v11 implementation was missing line 130.

firebase-ios-sdk/FirebaseAuth/Sources/Utilities/FIRAuthURLPresenter.m

Lines 127 to 137 in eca84fd

	- (void)safariViewControllerDidFinish:(SFSafariViewController *)controller {
	dispatch_async(FIRAuthGlobalWorkQueue(), ^() {
	if (controller == self->_safariViewController) {
	self->_safariViewController = nil;
	// TODO:Ensure that the SFSafariViewController is actually removed from the screen before
	// invoking finishPresentationWithURL:error:
	[self finishPresentationWithURL:nil
	error:[FIRAuthErrorUtils webContextCancelledErrorWithMessage:nil]];
	}
	});
	}

Fix #13645

[paulb777]

LGTM! Please add changelog