XWIKI-9414: Do not display "Subwikis directory" if there is no subwiki.

Trivial implementation.

It could be better to check if there is a workspace where the user has an access, but I guess it would have a higher cost and this template.vm is used everywhere.
XWIKI-9414: Do not display "Subwikis directory" if there is no subwiki.

* Replace $listtool.size($services.workspace.getWorkspaces())>1 by
  $services.workspace.getWorkspaces().size() > 1 since getWorkspaces()
  always returns a list.
Merge pull request #148 from gdelhumeau/XWIKI-9414

XWIKI-9414: Do not display "Wikis directory" if there is no subwiki