Fatal error "Call to a member function getPath() on null" when deleting an entity whose alias has a dependency

Problem/Motivation

Deleting a node (or any aliased entity) can trigger a fatal error when the Trash module is installed, or more generally whenever a `PathAliasDependency` row references a path alias that no longer exists:

Error: Call to a member function getPath() on null in Drupal\pathauto_update\PathAliasDependencyRepository->updatePathAliasesByType() (line 244 of modules/contrib/pathauto_update/src/PathAliasDependencyRepository.php).                                                                                                    

Steps to reproduce

1. Install trash alongside pathauto_update.
2. Have content whose alias is a dependency of another alias.
3. Delete (send to trash) a node. Trash soft-deletes the associated path alias, which re-fires an entity update; "updatePathAliasesByType()" then iterates the dependencies and hits a dependency whose alias has been removed.

(Not strictly Trash-specific, any state where a PathAliasDependency points at a deleted alias reproduces it, since `getPathAlias()` is documented to return NULL in that case.)

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes