Fix call to non-existing newEmptyValue() in PhabricatorDashboardPortalProfileMenuEngine

Summary:
PhabricatorDashboardPortalProfileMenuEngine calls $this->newEmptyValue() but neither PhabricatorDashboardPortalProfileMenuEngine nor its parent PhabricatorProfileMenuEngine define a newEmptyValue() method.
Thus use existing newNoContentView() in parent PhabricatorProfileMenuEngine (which according to its string output is supposed to cover this case).

Test Plan:
Go to a Portal with no content. Before applying the patch, Phorge shows the message defined in the parent class PhabricatorProfileMenuEngine: No Content; No visible menu items can render content.
After applying this change, it correctly shows the message defined in PhabricatorDashboardPortalProfileMenuEngine: No Portal Content; None of the visible menu items in this portal can render any content.

Reviewers: O1 Blessed Committers, speck

Reviewed By: O1 Blessed Committers, speck

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25648