Notice: Undefined index: #type in drupal_process_states()

Problem/Motivation

Problem

• When #states is attached to a form element that contains #markup the following notice occurs:
  

  Notice: Undefined index: #type in drupal_process_states() (line 583 of core/includes/common.inc).

Cause

• This happens because a #markup element does not require the #type to be set.

Proposed resolution

1. Fix the PHP notice.
2. Add a regression (kernel) test to ensure that this bug does not reappear in the future.

Notes

• There are no PHP/backend tests for #states right now, because the "functionality" on the PHP side is in essence just a json_encode(), so nothing worth to test.

Remaining tasks

• Make sure patch applies to 8.5.x and 8.6.x as well.
• Write an unit (kernel) test to ensure that the bug does not reappear in the future.

User interface changes

No.

API changes

No.

Data model changes

No