[TIMOB-23459] Android: Ti.Filesystem.getFile causes hard crash with null argument

In our mocha tests suite we used Ti.Filesystem.applicationDirectory to access files from the app Resources directory (xml files, js files, etc). Apparently this property is undefined/null on Android and we should instead be using Ti.Filesystem.resourcesDirectory. However, if we do something like:

Ti.Filesystem.getFile(Ti.Filesystem.applicationDirectory, 'app.js');

That code will hard-crash Android with an NPE. Considering that is valid code on iOS, and that we don't complain/spit out an error - but actually crash the app, I'd think we might want to safeguard a little better here. If we see a null in the path arguments, we should probably spit out an error and return null.