fix: return nil from filterManualImportToSplitTracks when NewFiles is empty

When item.Resp is nil or NewFiles is empty, the function was returning the
full unfiltered outputs slice. This made filterSplitOutputs always see a
non-empty result, making the filterManualImportToNumberedTracks fallback
completely unreachable (dead code).

Return nil instead to signal "couldn't filter", allowing the numbered track
pattern fallback to work as intended for CUE splits where MoveFiles has
already moved tracks back to the original folder.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: mprachar

pkg/unpackerr/lidarr.go

@@ -168,7 +168,7 @@ func crossPlatformBase(path string) string {
 // host (e.g. Linux) may use different paths than the Starr app (e.g. Windows).
 func filterManualImportToSplitTracks(outputs []*lidarr.ManualImportOutput, item *Extract) []*lidarr.ManualImportOutput {
 	if item == nil || item.Resp == nil || len(item.Resp.NewFiles) == 0 {
-		return outputs
+		return nil // Signal that we couldn't filter; caller should try fallback.
 	}
 
 	splitFiles := make(map[string]struct{}, len(item.Resp.NewFiles))