fix: crash when missing metadata.created

src/mp4.py

@@ -176,7 +176,10 @@ def write_metadata(song: bytes, metadata: SongMetadata, embed_metadata: list[str
         absolute_cover_path = cover_path.absolute()
         with open(cover_path.absolute(), "wb") as f:
             f.write(metadata.cover)
-    time = datetime.strptime(metadata.created, "%Y-%m-%d").strftime("%d/%m/%Y")
+    if metadata.created:
+        time = datetime.strptime(metadata.created, "%Y-%m-%d").strftime("%d/%m/%Y")
+    else:
+        time = ""
     subprocess.run(["mp4box", "-time", time, "-mtime", time, "-keep-utc", "-name", f"1={metadata.title}", "-itags",
                     ":".join(["tool=", f"cover={absolute_cover_path}",
                               metadata.to_itags_params(embed_metadata)]),