[auth] Update decoders for consistency with v10 nil behavior

[paulb777]

Decoding Initializer Comparison between Firebase 11 and Firebase 10: Firebase 10 allowed many properties to be decoded as nil. This PR restores much of that behavior.

Addresses comments in audit below except:

• Avoided changing public APIs: AuthDataResult, MultiFactorInfo, and AdditionalUserInfo
• 4 and 5: F11 will only decode more than F10 - like zero length strings
• 7 A non-nil accessToken is fundamental for SecureTokenService
• 8 A non-nil phoneNumber is fundamental for PhoneAuthCredential

[ncooke3]

Thanks!