Check bundle optional for localized string

[nrurnru]

resolve https://github.com/google/GoogleSignIn-iOS/issues/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. https://github.com/google/GoogleSignIn-iOS/blob/b45a83701141abde85a5e059a46b5ff039dca9ec/GoogleSignInSwift/Sources/GoogleSignInButtonStrings.swift#L41

[seyfeddin]

@brnnmrls Hi, this fixes a crash in our application when users try to login using a google workspace account and didn't set a passcode for the device. Only happens when GoogleSignIn is imported as an .xcframework.