Mongo query unit test fixed.

extensions/mongo/Collection.php

@@ -253,7 +253,7 @@ class Collection extends Object
 				$result[] = $actualValue;
 			} else {
 				$key = $this->normalizeConditionKeyword($key);
-				if (strncmp('$', $key, 1) !== 0 && array_key_exists(0, $actualValue)) {
+				if (strncmp('$', $key, 1) !== 0 && is_array($actualValue) && array_key_exists(0, $actualValue)) {
 					// shortcut for IN condition
 					$result[$key]['$in'] = $actualValue;
 				} else {

extensions/mongo/Query.php

@@ -123,9 +123,10 @@ class Query extends Component implements QueryInterface
 	 */
 	protected function buildCursor($db = null)
 	{
-		$where = $this->where;
-		if (!is_array($where)) {
+		if ($this->where === null) {
 			$where = [];
+		} else {
+			$where = $this->where;
 		}
 		$selectFields = [];
 		if (!empty($this->select)) {