[gitlab-api] how to get issues that have no milestone

Description

According to API doc, we can list issues like this:

GET /groups/:id/issues?milestone=1.0.0

But how to list issues that have no milestone? This does not work:

GET /groups/:id/issues?milestone=

Proposal

GET /groups/:id/issues?milestone=

Links / references

https://docs.gitlab.com/ee/api/issues.html#list-group-issues