Do not display nb_users series in row evolution is no data for the metric.

plugins/CoreHome/DataTableRowAction/RowEvolution.php

@@ -229,6 +229,14 @@ class RowEvolution
             list($first, $last) = $this->getFirstAndLastDataPointsForMetric($metric);
             $details = Piwik::translate('RowEvolution_MetricBetweenText', array($first, $last));
 
+            // TODO: this check should be determined by metric metadata, not hardcoded here
+            if ($metric == 'nb_users'
+                && $first == 0
+                && $last == 0
+            ) {
+                continue;
+            }
+
             if ($change !== false) {
                 $lowerIsBetter = Metrics::isLowerValueBetter($metric);
                 if (substr($change, 0, 1) == '+') {