Actually fix PHP 8.1 deprecation warning

Follow-up to 3.1.6 to actually fix the warning for real this time.

Fix PHP 8.1 deprecation warnings

This release fixes known PHP 8.1 deprecation warnings. See #4.

Improved validation to catch empty inputs

Improved validation in several grant types to catch and return relevant error messages for empty inputs (i.e. an empty string).

Use default generator for refresh tokens

Use the default token generator for refresh tokens.

Use default token generator for auth codes

Remove invalid & redundant namespaces. Use default token generator for generating codes in the authorisation code grant type.

Enable alternative access token generation strategy.

This release enables an alternative access token generation strategy to be used with grant types.