Fix PHP 8.4 implicit nullable deprecation warnings

Problem/Motivation

PHP 8.4 deprecates implicitly nullable parameters. When a parameter has a typed declaration (e.g., string) but has NULL as its default value, PHP 8.4 emits a deprecation warning:

Deprecated: Implicitly marking parameter $conversion as nullable is deprecated, 
the explicit nullable type must be used instead

Debug Report

1. Run any LDAP authentication test with PHP 8.4+
2. Observe deprecation warnings in the output

Affected files:

ldap_servers/src/Helper/ConversionHelper.php line 92:

public static function convertAttribute(string $value, string $conversion = NULL): string {

ldap_servers/tests/modules/ldap_servers_dummy/src/FakeLdap.php line 50:

public function bind(string $dn = NULL, string $password = NULL): void {

Proposed resolution

Add explicit nullable type declarations (?string) to affected parameters:

// ConversionHelper.php
public static function convertAttribute(string $value, ?string $conversion = NULL): string {

// FakeLdap.php
public function bind(?string $dn = NULL, ?string $password = NULL): void {

Remaining tasks

• Review and commit patch

User interface changes

None.

API changes

None - this is a backwards-compatible fix. The ?string nullable type accepts the same values as before (string or null).

Data model changes

None.