Add support for empty registry field in .ciux

[fjammes]

If registry field is empty/not defined, no image should be returned:

 fjammes@clrinfopo18  ~/src/github.com/k8s-school/demo-harbor   main ±  ciux ignite .
Project demo-harbor
  /home/fjammes/src/github.com/k8s-school/demo-harbor@33c8733
Dependencies:
  Package: github.com/k8s-school/[email protected]
  Package: github.com/k8s-school/[email protected]
Image:
/demo-harbor:33c8733, in registry: false

cat ~/.ciux/ciux.sh 
# Core project
export DEMO_HARBOR_DIR=/home/fjammes/src/github.com/k8s-school/demo-harbor
...
export CIUX_IMAGE_URL=/demo-harbor:33c8733