Fix issue where empty dir name in s3 key breaks listdir api.

Author: varunmittal91

Diff for: s3fs/core.py

@@ -690,7 +690,7 @@ async def _lsdir(
         if not prefix:
             prefix = ""
         if key:
-            prefix = key.lstrip("/") + "/" + prefix
+            prefix = key + "/" + prefix
         if path not in self.dircache or refresh or not delimiter or versions:
             try:
                 logger.debug("Get directory listing page for %s" % path)