fix(compose): honor LLAMA_SERVER_IMAGE override on apple overlay

[patil2001]

Summary

On multi-GPU NVIDIA installs whose assignment JSON lacks an entry for whisper/comfyui/embeddings, jq's .gpus[0]? prints the literal string null, which phase 03 persists verbatim — compose then renders device_ids: ["null"] and docker compose up fails container-create with could not select device "null". The three extractions now fall back to empty, matching how the CLI's own reassign path guards the same reads.

AI Assistance

AI-assisted trace from jq output to rendered compose. The author reviewed the guard style used elsewhere and is responsible for the final diff.

Release Lane

• Stable hotfix targeting release/2.6.x
• Mainline change targeting main
• Next-minor work targeting the next feature/minor release
• Not sure; reviewer should help classify

Stable hotfix reason:

Not targeting the stable release branch directly.

Changed Surface

• Docs only
• Tests only
• Dashboard UI
• Installer
• Core runtime
• Dashboard API
• CI workflows

Validation

• bash -n ods/installers/phases/03-features.sh - passed.