spec.py, AbstractFileSystem._ls_from_cache: Avoid looking up key in dircache twice (#1561)

---------

Co-authored-by: Martin Durant <[email protected]>

Author: KellyWalker

Diff for: fsspec/spec.py

@@ -358,8 +358,10 @@ def _ls_from_cache(self, path):
         but contains nothing), None if not in cache.
         """
         parent = self._parent(path)
-        if path.rstrip("/") in self.dircache:
+        try:
             return self.dircache[path.rstrip("/")]
+        except KeyError:
+            pass
         try:
             files = [
                 f