gh-153529: Fix zipfile.Path.is_symlink() on archive root

Author: whyvineet

Lib/zipfile/_path/__init__.py

@@ -414,6 +414,9 @@ def is_symlink(self):
         """
         Return whether this path is a symlink.
         """
+        if not self.at:
+            return False
+
         info = self.root.getinfo(self.at)
         mode = info.external_attr >> 16
         return stat.S_ISLNK(mode)