I am using automation to successfully flag an issue:

The field name for the flag is "Impeded". The field is Locked and is of the type is Checkboxes and only has 1 option: Impediment
I am now trying to create a rule for removing the flag but nothing is working:
I have tried with normal Edit Issue option with blank:

I have tried Edit Issue with Advanced but none of these options are working: empty string, null and empty list (tried them independently rather than all at once)
{
"fields": {
"Impeded": ""
}
}
{
"fields": {
"Impeded": null
}
}
{
"fields": {
"Impeded": []
}
}
Is this just not possible? Or do I need to change it some other way?