Fix of a potential bug found by ruff.

src/tscut/tscut.py

@@ -2983,9 +2983,12 @@ def extract_all_streams(ffmpeg_path:str, ffprobe_path:str, input_file:IO[bytes],
 # Merge a list of mkv files passed as input, and produce a new MKV as output
 @typechecked
 def merge_mkvs(mkvmerge_path:str, inputs: list[IO[bytes]], output_name:str,
-               concatenate: bool=True, timestamps: dict[int, IO[str]]={}) -> IO[bytes]|None:
+               concatenate: bool=True, timestamps: dict[int, IO[str]] | None = None) -> IO[bytes]|None:
     logger = logging.getLogger(__name__)
 
+    if timestamps is None:
+        timestamps = {}
+
     fds = []
     try:
         out = open(output_name, 'wb+')