Bug 447542 - NPE if no libhover docs given

- don't return a ICHelpDescriptor if the help docs
  aren't present for the extension

Change-Id: If87af603c8cdde9bd3d68faefe783ed29cd96a51
Reviewed-on: https://git.eclipse.org/r/35418
Tested-by: Hudson CI
Reviewed-by: Jeff Johnston <jjohnstn@redhat.com>
Tested-by: Jeff Johnston <jjohnstn@redhat.com>