Prevent access array offset of type null in EntityReference->query()

Problem/Motivation

Created "Entity reference display" in Views module can lead to a notice message because the code tryes to access an array offset that doesnt exists.

Steps to reproduce

- Create a view display of type 'entity reference'.
- Execute it programmatically
- you should see:

Notice: Trying to access array offset on value of type null in Drupal\views\Plugin\views\display\EntityReference->query() (line 195 of core/modules/views/src/Plugin/views/display/EntityReference.php).

Proposed resolution

Apply verification of "empty" before using.