Setting URL alias to "/" causes TypeError with Workspaces enabled

Problem/Motivation

Setting a page alias to "/" with Workspaces enabled causes a fatal 500 error on several pages:

TypeError: Drupal\workspaces\WorkspacesAliasManager::getAliasByPath(): Return value must be of type string, null returned in Drupal\workspaces\WorkspacesAliasManager->getAliasByPath() (line 49 of core/modules/workspaces/src/WorkspacesAliasManager.php).

WorkspacesAliasManager::getAliasByPath() returns AliasManager::getAliasByPath(), which can return null rather than a string in this circumstance.

Steps to reproduce

- Enable the workspaces module
- Set the URL alias on any page to "/"
- Save the page

Proposed resolution

Some combination of:
- Validate the URL alias field to ensure "/" is invalid
- Ensure that AliasManager::getAliasByPath() returns a string
- Ensure that AliasManager::getAliasByPath() returns the path if the alias lookup returns null/empty

This issue (https://www.drupal.org/project/drupal/issues/3100350) suggests that setting "/" may be desired in the future, so might be worth focusing on fixing the null response here.

Remaining tasks

Create MR for initial fix attempt

User interface changes

N/A

Introduced terminology

N/A

API changes

N/A

Data model changes

N/A