Do not have a fatal error on a missing action

[AronNovak]

#1015

What does this Pull Request do?

Makes PresetReaction more robust by not relying on the assumption that all actions are always available.

A brief description of what the intended result of the PR will be and/or what
problem it solves.

During using Islandora Drupal module inside https://github.com/Gizra/drupal-starter, we had a fatal error during the installation:

 [error]  Error: Call to a member function execute() on null in Drupal\islandora\PresetReaction\PresetReaction->execute() (line 59 of /var/www/html/web/modules/contrib/islandora/src/PresetReaction/PresetReaction.php) #0 /var/www/html/web/modules/contrib/islandora/src/IslandoraUtils.php(389): Drupal\islandora\PresetReaction\PresetReaction->execute(Object(Drupal\taxonomy\Entity\Term))

#1 /var/www/html/web/modules/contrib/islandora/islandora.module(226): Drupal\islandora\IslandoraUtils->executeTermReactions('\\Drupal\\islando...', Object(Drupal\taxonomy\Entity\Term))

#2 [internal function]: islandora_taxonomy_term_insert(Object(Drupal\taxonomy\Entity\Term))

#3 /var/www/html/web/core/lib/Drupal/Core/Extension/ModuleHandler.php(409): call_user_func_array(Object(Closure), Array)

What's new?

No new functionalities were added.

How should this be tested?

Intentionally creating a config that refers to a non-existing action would be sufficient.

Documentation Status

N/A

Interested parties

@Islandora/committers

[AronNovak]

The failures are not related to this change:

[joecorall]

Thanks for the PR. This LGTM.