fix(user_ldap): Ensure host is a string when checking if `ldapi`

Signed-off-by: Ferdinand Thiessen <opensource@fthiessen.de>

apps/user_ldap/lib/Configuration.php

 	 * Returns TRUE if the ldapHost variable starts with 'ldapi://'
 	 */
 	public function usesLdapi(): bool {
-		return (substr($this->config['ldapHost'], 0, strlen('ldapi://')) === 'ldapi://');
+		$host = $this->config['ldapHost'];
+		return is_string($host) && (substr($host, 0, strlen('ldapi://')) === 'ldapi://');
 	}
 }