Enhance CQL to search pages without labels.

Problem definition

In CQL, we can search for pages with specific labels with the below criteria.

https://xxxxx.atlassian.net/wiki/rest/api/content/search?cql=type=page and label =' some label'

However, it is not possible to search for pages that do not have labels.

Steps to replicate

1. Invoke Confluence REST API https://xxxxx.atlassian.net/wiki/rest/api/content/search?cql=type=page and label =' some label' on a space that has some labels. The query will return the page URLs.
2. Now modify the REST API to search for pages without labels.(try label='', label=null etc). The label criteria will be ignored in the query.

Suggested solution

Enhance CQL so that it can search pages without labels.