Remote Config: inaccurate nullability annotations

[maksymmalyhin]

In Firebase 8 the Remote Config API has the following inaccurate nullability annotations:

• [FIRRemoteConfig remoteConfig]. The return value is marked as nonnull but the implementation actually can return nil
• [FIRRemoteConfig remoteConfigWithApp:]. The return value is marked as nonnull but the implementation can return nil

The inaccurate nullability annotations prevent safe handling of the nil cases in Swift.

This is a breaking change, so will require a major version bump.

[ryanwilson]

We should likely throw an exception here to alleviate this:

firebase-ios-sdk/FirebaseRemoteConfig/Sources/FIRRemoteConfig.m

Lines 104 to 105 in d4c4e8c

	// TODO: Maybe throw an exception here? That'd be a breaking change though, but at this point
	// RC can't work as expected.

[ryanwilson]

Fixed in #9551 which is merged to the Firebase 9 branch.