Multiple issues detected when running on PHP 8

[vandevusse]

Succinct list of issues found so far as follows:

1. Helpers::keyExists
   $haystack can be passed as empty, causes in_array function to throw exception

2. UserRepository::isFavorite
   $favorites can be empty, causes in_array function to throw exception

3. SyncSingleFavorite::addFavorite
   $favorites[$key]['posts'][] = $this->post_id; causes the following error:
   Fatal error: [] operator not supported for strings
   I believe the issue here is that the $favorites[$key]['posts'] array is not initialized

Issue also occurs lower down at $favorites[$key]['groups'][$group_key]['posts']

[MacGyer]

Another one, introduced with 2.3.4:

PHP Deprecated: Optional parameter $site_id declared before required parameter $all_favorites is implicitly treated as a required parameter