fix ecs__enum crash on permission denied

[h00die]

Problem

When using ecs__enum without cluster list permissions, we get the following output:

Pacu (blackbox2:imported-arte) > run ecs__enum --region us-east-1
  Running module ecs__enum...
[ecs__enum] Starting region us-east-1...
[ecs__enum] FAILURE: 
[ecs__enum]   AccessDeniedException
[ecs__enum]     Skipping cluster enumeration...

[2024-09-12 16:28:04] Pacu encountered an error while running the previous command. Check /root/.local/share/pacu/blackbox2/error_log.txt for technical details, or use the debug command. [LOG LEVEL: MINIMAL]

    <class 'TypeError'>: 'NoneType' object is not subscriptable

Checking the error message we see:

[2024-09-12 16:18:03] (blackbox2): 
Traceback (most recent call last):
  File "/root/pacu/pacu/main.py", line 1855, in run_gui
    self.idle()
  File "/root/pacu/pacu/main.py", line 1711, in idle
    self.idle()
  File "/root/pacu/pacu/main.py", line 1711, in idle
    self.idle()
  File "/root/pacu/pacu/main.py", line 1711, in idle
    self.idle()
  [Previous line repeated 4 more times]
  File "/root/pacu/pacu/main.py", line 1709, in idle
    self.parse_command(command)
  File "/root/pacu/pacu/main.py", line 623, in parse_command
    self.parse_exec_module_command(command)
  File "/root/pacu/pacu/main.py", line 815, in parse_exec_module_command
    self.exec_module(command)
  File "/root/pacu/pacu/main.py", line 1032, in exec_module
    summary_data = module.main(command[2:], self)
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/root/pacu/pacu/modules/ecs__enum/main.py", line 116, in main
    for arn in response['clusterArns']:
               ~~~~~~~~^^^^^^^^^^^^^^^
<class 'TypeError'>: 'NoneType' object is not subscriptable

Solution

Generate the object which is expected, then gracefully handle accessing items that aren't there.

New output:

Pacu (blackbox2:imported-arte) > run ecs__enum --region us-east-1
  Running module ecs__enum...
[ecs__enum] Starting region us-east-1...
[ecs__enum] FAILURE: 
[ecs__enum]   AccessDeniedException
[ecs__enum]     Skipping cluster enumeration...
[ecs__enum]   0 cluster arn(s) found.
[ecs__enum]   0 container(s) found.
[ecs__enum]   0 service(s) found.
[ecs__enum] FAILURE: 
[ecs__enum]   AccessDeniedException
[ecs__enum]   Skipping instance enumeration...
[ecs__enum]   0 task definition(s) found.
[ecs__enum] ecs__enum completed.

[ecs__enum] MODULE SUMMARY:

  Regions:
     us-east-1

    0 total cluster(s) found.
    0 total container(s) found.
    0 total service(s) found.

[j0eblow]

tested and approved. lgtm, thanks