Support nonetype trivial default

[slothkong]

This is done to align with the precedence set by CLI tools like Terraform/OpenTofu, where setting null (conceptual equivalent of None) is needed to avoid the complexity of dealing with trivial default for values with a complex structure (i.e. dictionaries containing sub dictionaries, lists of dictionaries, etc.)

IMPORTANT: The ansible.builtin.assert statements used for input validation must continue allowing empty string ('') as a synonym of null. This is a technical limitation of the var_prompt statements we used on Ansible Playbooks that import this Ansible Role.
To be clear , at the time of writing, when an Ansible Playbook runs var_prompt and asks for an optional input , users have the option to press enter to skip, indicating they want to set the trivial default value. If users do press enter to skip, var_prompt set the corresponding variable as a empty str, NOT as a noneType. Hence the Ansible Role needs to deal with both as if they were equivalent. The same is true for the literal string containing the word None, which is set the the Ansible Playbook if run in non-interactive mode while ommitting the variable upon running Ansible.

cc: @pacospace