Drupal\Component\Plugin\Exception\PluginNotFoundException: The "0" plugin does not exist

Problem/Motivation

If the Events Logging configuration is saved without checking the "Database" checkbox in the Enabled events logging plugins section, a PluginNotFoundException is thrown when creating a taxonomy term

Steps to reproduce

1. Go to Events Logging settings page
2. Scroll to bottom and un-check "Database" in plugins section. Click Save
3. Go to Structure -> Taxonomy -> (select any taxonomy)
4. Click on "Add Term", fill out the form and Submit

When the settings form is saved without the plugin selected, the plugin value is returned as "database": "0". The EventLog class is getting the "0" value and expecting it to be a plugin ID. Attached patch addresses this issue.