fix(Server Backup Management): print valid JSON/YAML output for list cmds

[j1n-o9r]

Description

relates to STACKITCLI-386 / #893

Testing Instructions

Create a new server with name yyy and id zzz

• stackit server backup list --server-id zzz -> Expected: No backups found for server yyy

• stackit server backup list --server-id zzz --output-format json -> Should output valid JSON

• stackit server backup list --server-id zzz --output-format yaml -> Should output valid YAML

• stackit server backup schedule list --server-id zzz -> Expected: No backup schedules found for server yyy

• stackit server backup schedule list --server-id zzz --output-format json -> Should output valid JSON

• stackit server backup schedule list --server-id zzz --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)
• 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)