fixed PHP 7.2 compatibility (#190)

* fixed Warning: count(): Parameter must be an array or an object that implements Countable

Author: ondrakub

src/Database/Table/GroupedSelection.php

@@ -158,7 +158,7 @@ protected function execute(): void
 			foreach ((array) $this->rows as $key => $row) {
 				$ref = &$data[$row[$this->column]];
 				$skip = &$offset[$row[$this->column]];
-				if ($limit === null || $rows <= 1 || (count($ref) < $limit && $skip >= $this->sqlBuilder->getOffset())) {
+				if ($limit === null || $rows <= 1 || (count($ref ?? []) < $limit && $skip >= $this->sqlBuilder->getOffset())) {
 					$ref[$key] = $row;
 				} else {
 					unset($this->rows[$key]);