Adding to https://stackoverflow.com/users/1638814/nvartolomei answer, which will probably fix your error.
Strictly answering your question, I just want to point out that the when:
statement is probably correct, but would look easier to read in multiline and still fulfill your logic:
when:
- sshkey_result.rc == 1
- github_username is undefined or
github_username |lower == 'none'
https://docs.ansible.com/ansible/latest/user_guide/playbooks_conditionals.html#the-when-statement