Make AudioTrack.mimeType nullable to prevent Android playback stall

The POST /api/items/<id>/play/<episodeId> response inlines every episode
of a podcast with an audioTrack per episode. Server-side, audioTrack.mimeType
is derived from audioFile.mimeType, which is null when the row was created
without a successful audio probe (direct DB inserts, import scripts,
partial probe failures).

The Jackson deserializer hits one null mimeType among the inlined
episodes, throws MissingKotlinParameterException, and the entire
play-session response fails to deserialize. ExoPlayer is never called,
the UI silently stalls.

Make mimeType nullable on the data class to tolerate this. ExoPlayer's
MediaItem.Builder.setMimeType already accepts nullable. Defensively
coerce null to empty string for Cast's MediaInfo.Builder.setContentType,
whose Java signature is non-null.

Fixes #1905

android/app/src/main/java/com/audiobookshelf/app/data/AudioTrack.kt

@@ -10,7 +10,7 @@ data class AudioTrack(
         var duration: Double,
         var title: String,
         var contentUrl: String,
-        var mimeType: String,
+        var mimeType: String?,
         var metadata: FileMetadata?,
         var isLocal: Boolean,
         var localFileId: String?,

android/app/src/main/java/com/audiobookshelf/app/data/PlaybackSession.kt

@@ -332,7 +332,7 @@ class PlaybackSession(
             MediaInfo.Builder(mediaUri.toString())
                     .apply {
                       setContentUrl(mediaUri.toString())
-                      setContentType(audioTrack.mimeType)
+                      setContentType(audioTrack.mimeType ?: "")
                       setMetadata(castMetadata)
                       setStreamType(MediaInfo.STREAM_TYPE_BUFFERED)
                     }