MediaPlayerAPI: calling it without any argument could crash the API app

i.e. `$PREFIX/libexec/termux-api MediaPlayer`

The API would crash because:
```
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.String.toUpperCase()' on a null object reference
```

Note that it would probably crash even if `command` is `switch`ed without `toUpperCase`.

Btw, what's the point of the `toUpperCase` anyway?

@dkramer95 