- Preserve empty string environment variables instead of converting to null
Empty strings and null have different semantics in Docker Compose:
* Empty string (VAR: ""): Variable is set to "" in container (e.g., HTTP_PROXY="" means "no proxy")
* Null (VAR: null): Variable is unset/removed from container environment
- Remove empty top-level sections (volumes, configs, secrets) from generated compose files
These sections now only appear when they contain actual content, following Docker Compose best practices
- Add safety check for missing volumes in validateComposeFile to prevent iteration errors
- Add comprehensive unit tests for both fixes
Fixes#7126
