Accessibility issue with category dropdown

**Describe the bug**
When running an accessibility audit on a wordpress page using the WP Job Manager shortcode "[jobs per_page="10" job_types="Full Time, Part Time, Temporary"]", the dropdown menu for Categories triggers a accessibility issue in the audit. Audit error message is:

_Elements with [role] that require specific children [role]s, are missing.
Some ARIA parent roles must contain specific child roles to perform their intended accessibility functions._

**Failing Elements**
span.select2-selection.select2-selection--single

This element has aria-labelledby assigned to it, but it points to another element that doesn't exist on the page.

**To Reproduce**
Steps to reproduce the behavior:
1. Go to page using the shortcode.
2. Open Dev Tools and run an audit.
3. See error in audit summary.

**Expected behavior**
The element should either a) have its own aria-label, or b) the missing element needs to be put back in the html code with an appropriate label.

**WordPress Environment**
- WordPress Version: 5.2.3
- WP Job Manager Version: 1.33.5
- PHP Version: tested with 5.6.4 and 7.3

Reproduce the issue on a WordPress page using the [jobs per_page="10" job_types="Full Time, Part Time, Temporary"] shortcode, then inspect the Categories dropdown markup in DevTools. Start with the span.select2-selection.select2-selection--single element and its aria-labelledby target; done means the audit no longer reports a missing label target and the control has an appropriate accessible label.

Written by the indexing model from the issue text.