build credentials only if not null; close #8
build credentials only if not null; close #8

lwhiteley/react-native-facebook-login,leonacky/react-native-facebook-login,lwhiteley/react-native-facebook-login,brysgo/react-native-facebook-login,lwhiteley/react-native-facebook-login,brysgo/react-native-facebook-login,leonacky/react-native-facebook-login,leonacky/react-native-facebook-login,brysgo/react-native-facebook-login