setting path alias to `/` causes PHP8.1/str_starts_with/null error

Added `if (!$path) $path = '';` in `core/modules/path_alias/src/PathProcessor/AliasPathProcessor.php`.

This seems like something the path module was not intended to do and it should perhaps reject a single slash on validation.

Nevertheless for this to be considered we need to have test coverage of this proposed behavior change.

If this not intended, than setting a node as home/front page in system settings should change the canonical/alternate links automatically.

Related issues:

* url() should return / when asked for the URL of the frontpage 🐛 url() should return / when asked for the URL of the frontpage Needs work
* Unable to save '/' root path alias 🐛 Unable to save '/' root path alias Postponed (duplicate)
* Front Page canonical is Incorrect → (duplicate)

This issue still persists, because PHP8 deprecated null values in string functions, which will become errors instead of notices sooner or later.

To "fix" my initial motivation to set the path alias of my front page to /, I might install the Metatag module → . I'm not happy with adding 1,9 MiB/522 files/231 sub-folders just to set the correct canonical and shortlink tags, but it might be a workaround to get started.