Fix issue where tester.groups can be undefined

[aalej]

Description

Fixes #8191

Checking API explorer in https://firebase.google.com/docs/reference/app-distribution/rest/v1/projects.testers/list. Executing the method returns a JSON response of:

{
  "testers": [
    {
      "name": "projects/PROJECT_NUMBER/testers/tester_0@google.com",
      "groups": [
        "projects/PROJECT_NUMBER/groups/qa",
        "projects/PROJECT_NUMBER/groups/developers"
      ],
      "lastActivityTime": "2024-11-14T13:10:19.256962Z"
    },
    {
      "name": "projects/PROJECT_NUMBER/testers/tester_1@gmail.com",
      "lastActivityTime": "2025-01-30T14:03:28.146030Z"
    }
  ]
}

It looks like testers not part of any groups return a Tester object that does not have a groups field

Scenarios Tested

Run firebase appdistribution:testers:list --project PROJECT_ID

$ firebase appdistribution:testers:list --project PROJECT_ID
(node:29657) [DEP0040] DeprecationWarning: The `punycode` module is deprecated. Please use a userland alternative instead.
(Use `node --trace-deprecation ...` to show where the warning was created)
✔ Preparing the list of your App Distribution testers
┌───────────────────────────┬──────────────┬──────────────────────────────────────────────────────────────┬───────────────┐
│ Name                      │ Display Name │ Last Activity Time                                           │ Groups        │
├───────────────────────────┼──────────────┼──────────────────────────────────────────────────────────────┼───────────────┤
│ tester_0@google.com │              │ Thu Nov 14 2024 21:10:19 GMT+0800 (Philippine Standard Time) │ developers;qa │
├───────────────────────────┼──────────────┼──────────────────────────────────────────────────────────────┼───────────────┤
│ tester_1@gmail.com      │              │ Thu Jan 30 2025 22:03:28 GMT+0800 (Philippine Standard Time) │               │
└───────────────────────────┴──────────────┴──────────────────────────────────────────────────────────────┴───────────────┘
✔  Testers listed successfully

Sample Commands

[joehan]

Thanks for the quick fix!