lint: Restore pycodestyle wrapper’s check for empty list of files.

Commit ab647abad3 (#15779) accidentally
removed this.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

tools/linter_lib/pep8.py

@@ -5,4 +5,6 @@ from zulint.printer import colors
 
 
 def check_pep8(files: List[str]) -> bool:
+    if not files:
+        return False
     return run_command("pep8", next(colors), ["pycodestyle", "--", *files]) != 0