Handle case for invalid fighter name

[Hernanvb]

This will prevent the app from sending an 'undefined' query to request causing it to crash.
Instead the API will return fighter 'null' so any app can handle that gracefully.

[valish]

Thanks for finding this @Hernanvb -- I addressed the fighter name issue in the sherdog package, so the names are coming in correctly again. Let me know if your problem still persists.