FIROAuthCredential nullability of properties are wrong.

[ryanwilson]

[REQUIRED] Step 2: Describe your environment

• Xcode version: All
• Firebase SDK version: 4.0+
• Library version: 4.0+
• Firebase Product: auth

[REQUIRED] Step 3: Describe the problem

FIROAuthCredential has incorrect nullability for IDToken and accessToken. The constructor accepts a nullable NSString * for each, but the property is implicitly nonnull.

[ryanwilson]

Note: This would surface a breaking change for Swift users if it is changed if these are publicly surfaced, which I don't actually think they are at the moment.