Return null for getItem and removeItem of non-existent keys

[andrewzey]

Resolves https://github.com/robwalkerco/redux-persist-filesystem-storage/issues/47

Using in our react-native project.

Confirmed that The API mirrors AsyncStorage:

(async () => {
  console.log('asyncStorage getItem: ', await AsyncStorage.getItem('foo'));
  console.log('filesystemStorage getItem: ', await FilesystemStorage.getItem('foo'));
  console.log('asyncStorage removeItem: ', await AsyncStorage.removeItem('bar'));
  console.log('filesystemStorage removeItem: ', await FilesystemStorage.removeItem('bar'));
})();

getItem checks for existance in the catch block to reduce unneeded file system access.

removeItem could not do the same, as unlink doesn't throw when the file doesn't exist, it just returned undefined (whereas AsyncStorage returns null)

[robwalkerco]

Thanks for your PR @andrewzey

[andrewzey]

Glad to help!