Validate that the parameters exists before use it in array_unshift() function

Problem/Motivation

Once you go to edit the content type configuration from the module configuration page you will see in the logs this error:

Warning: array_unshift() expects parameter 1 to be array, null given in node_revision_delete_form_node_type_form_alter() (line 180 of /Users/adriancid/Sites/drupal83-test/modules/contrib/node_revision_delete/node_revision_delete.module)

Proposed resolution

Validate that the parameter exists.

Remaining tasks

None.

User interface changes

None.

API changes

None.

Data model changes

None.