How to specify No Value for a key in Ansible yaml?

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I’m a newbie in Ansible and I’d like to use the aws_ec2 plugin to return a list of ec2 instances with 2 tags: env=stage and a=(empty). I’m using “filters” with “tag” like the followings:

filters:
tag:Env:
- stage
tag:a:
-

Besides leaving a blank, I’ve also tried “”, ‘’, None, _, null, ~, but they all failed to return the list. I’ve also tried using the “tag-key” “filter” but no luck:

filters:
tag-key:
- a

Can someone please help? Also, would you also use filters?

Thanks.