Fix getting group '0' from database backend

Fix #9972

lib/private/group/database.php

@@ -185,7 +185,7 @@ class OC_Group_Database extends OC_Group_Backend {
 	public function groupExists($gid) {
 		$query = OC_DB::prepare('SELECT `gid` FROM `*PREFIX*groups` WHERE `gid` = ?');
 		$result = $query->execute(array($gid))->fetchOne();
-		if ($result) {
+		if ($result !== false) {
 			return true;
 		}
 		return false;

lib/private/group/manager.php

@@ -125,7 +125,7 @@ class Manager extends PublicEmitter {
 	 * @return \OC\Group\Group
 	 */
 	public function createGroup($gid) {
-		if (!$gid) {
+		if ($gid === '' || is_null($gid)) {
 			return false;
 		} else if ($group = $this->get($gid)) {
 			return $group;