fix(iaas): print valid JSON/YAML output for list cmds

[j1n-o9r]

Description

relates to STACKITCLI-241 / #893

Testing Instructions

1. With no server present in your project

• stackit server list -> Expected: No servers found for project "xxx"
• stackit server list --output-format json -> Should output valid JSON
• stackit server list --output-format yaml -> Should output valid YAML

2. With no network present in your project

• stackit network list -> Expected: No networks found for project "xxx"
• stackit network list --output-format json -> Should output valid JSON
• stackit network list --output-format yaml -> Should output valid YAML

3. With no network-area present in your organization

• stackit network-area list --organization-id xxx -> Expected: No servers found for project "xxx"
• stackit network-area list --organization-id xxx --output-format json -> Should output valid JSON
• stackit network-area list --organization-id xxx --output-format yaml -> Should output valid YAML

4. Create a new network and use its ID for the following commands (Note: Was already resolved in feat(iaas): List NICs for Project #1242 but since it is part of STACKITCLI-241 I leave the test instructions here.)

• Prerequisit: stackit network create --name xxx
• stackit network-interface list --network-id yyy -> Expected: No network-interface found for project "xxx"
• stackit network-interface list --network-id yyy --output-format json -> Should output valid JSON
• stackit network-interface list --network-id yyy --output-format yaml -> Should output valid YAML

Checklist

• Issue was linked above
• Code format was applied: make fmt
• Examples were added / adjusted (see e.g. here) (Note: Nothing to do)
• Docs are up-to-date: make generate-docs (will be checked by CI)
• Unit tests got implemented or updated
• Unit tests are passing: make test (will be checked by CI)
• No linter issues: make lint (will be checked by CI)