Filter out null and empty environment variables when generating Nixpacks build
configuration to prevent JSON parsing errors. Environment variables with null or
empty values were being passed as `--env KEY=` which created invalid JSON with
null values, causing deployment failures.
This fix ensures only valid non-empty environment variables are included in both
user-defined and auto-generated COOLIFY_* environment variables.
🤖 Generated with Claude Code
Co-Authored-By: Claude <noreply@anthropic.com>
