[google_sign_in_ios] Fix "callee requires a non-null parameter" analyzer warning

[jmagman]

The normal Cocoa pattern is to check a result, and only if it's nil then do something with the error.

https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/ErrorHandlingCocoa/CreateCustomizeNSError/CreateCustomizeNSError.html

Important: Success or failure is indicated by the return value of the method. Although Cocoa methods that indirectly return error objects in the Cocoa error domain are guaranteed to return such objects if the method indicates failure by directly returning nil or NO, you should always check that the return value is nil or NO before attempting to do anything with the NSError object.

Use this pattern with the result and error from the sign in completion block. This means -didSignInForUser is only called for non-nil users (the possibility of a nullable parameter was kicking off the analyzer warning).

Fixes flutter/flutter#153587

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[jmagman]

Removed, if user.idToken is nil then user.idToken.tokenString will also be nil. This is another normal Cocoa pattern, don't check nils since calling a selector (method) on nil will harmlessly also return nil.

[jmagman]

Swap to dot-notation because absoluteString is a property. This is just a convention, there's no functional difference.
https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/CocoaFundamentals/CocoaObjects/CocoaObjects.html

[stuartmorgan-g]

LGTM