Handle cases where there is no output from the check command

According to the docs `TimeoutExpired.output` may be `None` if there was no
output from the command.

Author: JamesWrigley

extra_data/cli/check_readable.py

@@ -113,7 +113,10 @@ def monitor_access_check(path):
                 timeout=args.timeout, capture_output=True,
                 env={'OMP_NUM_THREADS': '1'})
         except subprocess.TimeoutExpired as e:
-            path_states[path] = e.stdout.decode().splitlines()[-1]
+            if e.stdout is None:
+                path_states[path] = "Status unknown, no output from check command"
+            else:
+                path_states[path] = e.stdout.decode().splitlines()[-1]
             return 'T'
         else:
             path_states[path] = p.stdout.decode().splitlines()[-1]
@@ -158,4 +161,4 @@ def monitor_access_check(path):
 
 
 if __name__ == '__main__':
-    sys.exit(main())
+    sys.exit(main())