Give a warning when the cmslocation of a virtualhostgroup is blank

When the hst:cmslocation property is set on a hst:virtualhostgroup but is left blank, the channel manager does not work anymore. In org.hippoecm.hst.core.hosting.CustomMountAndVirtualCmsHostAugmenter#augment the split of the empty string creates a NPE (cmsCustomMountHostName.split("
.")). The icon of the CM perspective in the cms is missing, and all channels miss the configuration that is stored on the channels.

I think adding a check for the NPE and logging a warning would be an improvement.