Fix for UnboundLocalError: cannot access local variable 'resp' ...

[andyjsharp]

As described in issue #821, running the file-delete RPC can return module failure output that includes:

• No start of json char found
• UnboundLocalError: cannot access local variable 'resp' ...

This pull request fixes both issues by initializing resp before RPC execution.

In addition, some RPC operations such as file-delete may return an empty reply body. Previously this could lead to Unexpected response type <class 'NoneType'>. This change adds explicit handling for an empty file-delete reply so the task is treated as successful, without requiring failed_when: false.

Using the playbook from issue #821, with the following playbook change:

    - name: DELETE file from device (RPC file-delete)                                           
      juniper.device.rpc:                                                                       
        rpc: file-delete                                                                        
        kwargs:                                                                                 
          path: "{{ jmuse_remote_path }}"                                                       
      register: jmuse_delete_result                                                             
      changed_when: true 

The playbook now runs without failures:

TASK [PUT file to device] ********************************************************************
changed: [srx]

TASK [DELETE file from device (RPC file-delete)] *********************************************
changed: [srx]

TASK [Verify file no longer exists] **********************************************************
ok: [srx]

TASK [Show RPC delete summary] ***************************************************************
ok: [srx] => {
    "msg": {
        "contains_json_start_error": false,
        "contains_unbound_local_error": false,
        "delete_exception": "",
        "delete_failed": false,
        "delete_module_stderr": "",
        "delete_msg": "The RPC executed successfully with an empty reply.",
        "put_failed": false,
        "remote_path": "/var/tmp/jmuse-dev-bug-rpc-20260506T093003.txt",
        "verify_output": "\n/var/tmp/jmuse-dev-bug-rpc-20260506T093003.txt: No such file or directory\n"
    }
}