Allow in the runner register CLI to set default image to None.

Description

In the CLI for registering new runners you have to enter a default image

Please enter the default Docker image (e.g. ruby:2.6):

Later you can set image = "" in the config.

Usecase

This would allow forcing users to specify an image in their CI setup.
A current workaround is to use hello-world or scratch as default, so pipelines fail with a cryptic error message. If you manually set it to None, you get

ERROR: Preparation failed: No Docker image specified to run the build in

Which is exactly why the job (correctly) failed.

Proposal

Allow skipping Please enter the default Docker image (e.g. ruby:2.6): e.g.

Please enter the default Docker image (e.g. ruby:2.6); hit "enter" to skip: