Through the register keyword you assign a variable. You want to know what variable type it is (e.g., a string, a dictionary or a JSON). You know the data type is not a Boolean. What do you do?
If you have a variable (e.g., foobar) and you want to know what data type it is, do this:
when: foobar.stdout == ""
The playbook will tell you if foobar is a dictionary with an error. If there is no error, it is a string. If foobar is a JSON you will see a message when you run the playbook about "ansible.utils.unsafe.proxy.AnsibleUnsafeText"
(The above will not help you if it is a Boolean. You should compare it to "TRUE" or "FALSE" instead of "" if you think the value may be a Boolean.)