Return a default user record if json is broken

It's possible that json_decode returns null. Mostly the json is broken.
AddMissingDefaultValues expects an array. Pass null will fail.

Signed-off-by: Daniel Kesselberg <mail@danielkesselberg.de>

lib/private/Accounts/AccountManager.php

@@ -135,6 +135,9 @@ public function getUser(IUser $user) {
 		}
 
 		$userDataArray = json_decode($result[0]['data'], true);
+		if ($userDataArray === null || json_last_error() !== JSON_ERROR_NONE) {
+			return $this->buildDefaultUserRecord($user);
+		}
 
 		$userDataArray = $this->addMissingDefaultValues($userDataArray);