Use empty string instead of null

Signed-off-by: Julius Härtl <jus@bitgrid.net>

lib/private/Accounts/AccountManager.php

@@ -330,7 +330,7 @@ class AccountManager implements IAccountManager {
 	private function parseAccountData(IUser $user, $data): Account {
 		$account = new Account($user);
 		foreach($data as $property => $accountData) {
-			$account->setProperty($property, $accountData['value'], $accountData['scope'], $accountData['verified']);
+			$account->setProperty($property, $accountData['value'] ?? '', $accountData['scope'], $accountData['verified']);
 		}
 		return $account;
 	}