fix: return request output format when list is empty

[craicoverflow]

Fixes #579

Verification Steps

Run rhoas kafka list, rhoas kafka topic list, rhoas serviceaccount list and rhoas kafka consumergroup list, passing -o json and -o yml (requires 2 runs).

When there are no items, the JSON or YAML will be returned.

$ ./rhoas kafka list -o yaml
kind: KafkaRequestList
page: 1
size: 0
total: 0
items: []

$ ./rhoas kafka topic list -o json
{
  "count": 0,
  "items": [],
  "limit": 0,
  "offset": 0
}

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• Documentation added for the feature
• CI and all relevant tests are passing
• Code Review completed
• Verified independently by reviewer