Fix divison by 0 issue in debug plugin

If you send an empty DB query (thus strlen($query) = 0) you get a division by 0.

plugins/system/debug/debug.php

@@ -747,7 +747,7 @@ protected function displayQueries()
 
 				// How heavy should the string length count: 0 - 1.
 				$ratio = 0.5;
-				$timeScore = $queryTime / (strlen($query) * $ratio) * 200;
+				$timeScore = $queryTime / ((strlen($query) + 1) * $ratio) * 200;
 
 				// Determine color of bargraph depending on query speed and presence of warnings in EXPLAIN.
 				if ($timeScore > 10)