We've got an error on the dashboard, for the component which displays the number of visitor in a graph.
We get the error _Fatal error: Call to a member function getFirstRow() on a non-object in /PathToWWW/piwik/plugins/VisitsSummary/Controller.php on line 115_
It sometimes happen, sometimes not and we don't know why.
let's simply catch this empty datatable use case & return "No data" when the this happens, instead of php error.
(In ) Fixes #3296, check for empty datatable in VisitsSummary_Controller::setSparklinesAndNumbers.
Note: I couldn't reproduce the bug, but  should fix it.