shellcheck_run_steps: Avoid failure when YAML contains no run steps

Signed-off-by: dann frazier <dann.frazier@chainguard.dev>

Author: dannf

pre_commit_hooks/shellcheck_run_steps.py

@@ -49,6 +49,8 @@ def do_shellcheck(
                     ),
                 ),
             )
+        if len(all_steps) == 0:
+            return
         for step, shfile in all_steps:
             shfile.write(step["runs"])
             shfile.close()