Zero steps

[laszlocph]

With skip_clone: true introduced, there are cases where there are no pipeline steps to run.

Clone always ran in the past, now if we skip clone and the pipeline steps have branch/event conditions preventing steps from running, we can have cases where there are no steps to run. This is breaking the UI and the scheduler too.

This PR handles two cases:

• when a pipeline in multi-pipeline setup have no steps to run, that pipeline is not created at all
• in a single pipeline setup if there are no steps to run, the pipeline is not triggered at all, leaving no sign of the webhook in Drone, except informing the webhook sender about this special case