RN: Fix NSInvalidArgumentException for Invalid Font Family Name
RN: Fix NSInvalidArgumentException for Invalid Font Family Name

Summary:
We observed that in certain production scenarios, `[UIFont fontNamesForFamilyName:familyName]` returns `nil`.

This is problematic because we cannot insert `nil` (which is not an object type) into `NSCache`. Currently, this is causing `NSInvalidArgumentException` to be thrown.

This fix works around the problem by guarding against `nil`.

Changelog:
[iOS][Fixed] - Fix NSInvalidArgumentException for invalid font family names.

Reviewed By: fkgozali

Differential Revision: D31011394

fbshipit-source-id: a9eb9ce69efd832acca65787c665fcbb7b42a795

facebook/react-native,javache/react-native,javache/react-native,facebook/react-native,javache/react-native,janicduplessis/react-native,janicduplessis/react-native,javache/react-native,janicduplessis/react-native,janicduplessis/react-native,javache/react-native,janicduplessis/react-native,facebook/react-native,janicduplessis/react-native,facebook/react-native,janicduplessis/react-native,janicduplessis/react-native,javache/react-native,facebook/react-native,facebook/react-native,facebook/react-native,javache/react-native,javache/react-native,javache/react-native,facebook/react-native,facebook/react-native