Explicitly mark $directory as nullable in otp function to resolve PHP 8.1+ deprecation warning

[SoftHeroes]

Description:
This PR fixes a deprecation warning in PHP 8.1+ caused by the implicit nullable parameter $directory in the otp function. The warning occurs because PHP now requires explicitly marking nullable parameters with ?.

Changes Made:

• Updated the otp function signature in src/Helper.php from:

  function otp(string $directory = null): Otp

  to:

  function otp(?string $directory = null): Otp

• This ensures compatibility with PHP 8.1+ and prevents deprecation warnings.

Why This Is Needed:

• Without this change, users see a deprecation warning when running the package on PHP 8.1+.
• This update does not affect backward compatibility with older PHP versions.
• It aligns the function with best practices for defining nullable parameters.

Testing & Verification:

• Ensured that the function still works as expected with and without the $directory parameter.
• Checked compatibility with PHP versions prior to 8.1.