clean-slate-deployment should behave gracefully if there is nothing to clean

[meysholdt]

Is your feature request related to a problem? Please describe

Right now, the deploy-to-preview-environments on main fails, because the job has clean-slate-deployment enabled but there is no old preview environment to clean.

Example: https://werft.gitpod-dev.com/job/gitpod-build-main.3090

Describe the behaviour you'd like

Don't fail if clean-slate-deployment is enabled but there is nothing to clean.

Describe alternatives you've considered

Additional context

[mads-hartmann]

Latest build on main succeeded https://werft.gitpod-dev.com/job/gitpod-build-main.3092 ☺️