fix TS signature in TrackPlayer.getCurrentTrack()

TrackPlayer returns `null` on ios when current index is out of bounds/invalid (https://github.com/doublesymmetry/react-native-track-player/blob/f6229d68c2cdb650b90c38fa47f7e40d0028dfee/ios/RNTrackPlayer/RNTrackPlayer.swift#L608-L613)

src/trackPlayer.ts

@@ -266,7 +266,7 @@ async function getQueue(): Promise<Track[]> {
 /**
  * Gets the index of the current track.
  */
-async function getCurrentTrack(): Promise<number> {
+async function getCurrentTrack(): Promise<number | null> {
   return TrackPlayer.getCurrentTrack()
 }