Do not fail when there are no plugins to work with

[csemrau]

I have a new repository, where all the plugins are still on feature branches. Building master on that repo fails. The repository is configured to build only referenced plugins. In this case, the builds contains no plugins at all, which is quite unexpected for the assets compiler. A software build of master will likely fail later (because not even the platform plugin is there), but it should not be the assets compiler's fault.