[Auth] Add default values for decoding nil properties in UserInfoImpl

[ncooke3]

Address #14011 (comment)

If a UserInfoImpl instance fails to decode, the entire [String: UserInfoImpl] collection (below snippet) will fail to decode, which will make the user.providerData property nil.

firebase-ios-sdk/FirebaseAuth/Sources/Swift/User/User.swift

Lines 1736 to 1737 in 2176e99

	let providerData = coder.decodeObject(of: classes, forKey: kProviderDataKey)
	as? [String: UserInfoImpl]

The below snippet is the GetAccountInfoResponseProviderUserInfo response object for an account with email and phone providers enabled. The phone provider does not have a federatedID and therefore its corresponding UserInfoImpl instance does not have a uid. It's possible a more useful default could be given, but will have to further investigate this. cc: @Xiaoshouzi-gh

Falling back to "" is slightly safer than the ObjC implementation, that would bridge to an implicit nil because of UserInfo's non-null uid interface (UserInfoImpl conforms to UserInfo).

[paulb777]

Thanks!

Would you update the release note to say this part of the fix will be in the 11.6.0 source distros and 11.7.0 binary distros