Check bundle optional for localized string

[nrurnru]

resolve #309

Add optional check for bundle of framework.
In this case UIAlertAction occurred crash because localizedStringForKey returned nil.

Swift version already has check this way.

GoogleSignIn-iOS/GoogleSignInSwift/Sources/GoogleSignInButtonStrings.swift

Line 41 in b45a837

guard let frameworkBundle = Bundle.gidFrameworkBundle() else { return text }