Use `is not None` instead of `!= null`.

Fixes this error seen at the gate:


2024-02-15 15:19:20.880299 | TASK [handle-constraints-url : GET the constraints file]
2024-02-15 15:19:20.987221 | focal-medium | ERROR
2024-02-15 15:19:20.987644 | focal-medium | {
2024-02-15 15:19:20.987718 | focal-medium |   "msg": "The conditional check 'constraints_file is defined and pip_constraints_url is defined and pip_constraints_url != null and pip_constraints_url|length > 0' failed. The error was: error while evaluating conditional (constraints_file is defined and pip_constraints_url is defined and pip_constraints_url != null and pip_constraints_url|length > 0): 'null' is undefined\n\nThe error appears to be in '/var/lib/zuul/builds/e46ddf012092472a9aa826d844e4117d/trusted/project_0/github.com/openstack-charmers/zosci-config/roles/handle-constraints-url/tasks/main.yaml': line 8, column 3, but may\nbe elsewhere in the file depending on the exact syntax problem.\n\nThe offending line appears to be:\n\n\n- name: GET the constraints file\n  ^ here\n"
2024-02-15 15:19:20.987780 | focal-medium | }

https://openstack-ci-reports.ubuntu.com/artifacts/e46/908775/3/check/jammy-zed/e46ddf0/job-output.txt

openstack-charmers / zosci-config

Use `is not None` instead of `!= null`. #321