[TASK] Prepare towards PHP 8.4 compatibility

[sbuerk]

• [TASK] Avoid implicitly nullable class method parameter
  With PHP 8.4 marking method parameter implicitly nullable
  is deprecated and will emit a E_DEPRECATED warning. One
  recommended way to resolve this, is making it explicitly
  nullable using the ? nullable operator or adding a null
  type to an union type definition. [1]

  This prepares the way towards PHP 8.4 compatibility.

  [1] https://php.watch/versions/8.4/implicitly-marking-parameter-type-nullable-deprecated

• [TASK] Ensure coding-style for nullable type declarations
  This change modifies the php-cs-fixer ruleset and
  activate two rules related to nullable types.

  Following options are added:

    'nullable_type_declaration' => [
      'syntax' => 'question_mark',
    ],
    'nullable_type_declaration_for_default_null_value' => true,

  nullable_type_declaration ensures to use ?<type> declaration
  instead of union type <type>|null as a convention for properties,
  method and return types. For already union types the nullable is
  added as additional null union type ObjectOne|ObjectInterface|null.

  nullable_type_declaration_for_default_null_value ensures
  to use nullable type declarations to mitigate implicitly
  nullable method arguments [1] which has been already fixed
  with a series of dedicated changes.

  Used command(s):

  Build/Scripts/runTests.sh -s cgl
  

  [1] https://php.watch/versions/8.4/implicitly-marking-parameter-type-nullable-deprecated

[achimfritz]

👍 thanks