HLS: Skip merging continuity if all segments were skipped

If all segments of a continuity is skipped, i.e. by OnSegmentFilter, then this code fails as the folder wouldn't exist.

devine/core/manifests/hls.py

@@ -325,6 +325,7 @@ class HLS:
                             last_speed_refresh = now
                             download_sizes.clear()
 
+            if discontinuity_save_dir.exists():
                 with open(discontinuity_save_path, "wb") as f:
                     for segment_file in sorted(discontinuity_save_dir.iterdir()):
                         segment_data = segment_file.read_bytes()