[iOS] Don't trigger string_has_extra_arguments when original string is missing

[marcelofabri]

Looks like string_has_extra_arguments can be triggered if a string with arguments is present in a translated strings file, but missing from the original one.

~IMO this should be reported with a new "problem" type: extra_key_in_translation instead~ Looks like Android already has KeyMissingFromBase -- we should use it for iOS as well

This is happening for iOS, ~not sure if the same issue is present on Android~.

[marcelofabri]

I think this is actually the same as https://github.com/Asana/locheck/issues/42?