Fix registry optional run opts

What does this Merge Request do?

Fixes a problem where the empty string may get passed as a positional argument to docker, causing it to fail to start with an error of the form

docker: invalid reference format.
See 'docker run --help'.

Probably caused by an incorrect fix to a shellcheck violation: f089a3c7

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.