Fix HipChat Server

Fixes a regression in the HipChat service.

This regression occurs when the HipChat server url is not specified (which should default to HipChat cloud service). The property is sometimes not registering as `nil` so added a `blank?` check.

Also, changed the placeholder on server url because it could potentially be confusing. 

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/932