Make create-pr step condition not always false

[tautschnig]

Description of changes:

Fixup for 55d1557. I had failed to test both branches of the if-condition.

Resolved issues:

n/a

Related RFC:

n/a

Call-outs:

n/a

Testing:

• How is this change tested? Tested in my fork: https://github.com/tautschnig/kani/actions/runs/4706689843/jobs/8348087165

• Is this a refactor change? No

Checklist

• Each commit message has a non-empty body, explaining why the change was made
• n/a Methods or procedures are documented
• Regression or unit tests are included, or existing tests cover the modified code
• My PR is restricted to a single feature or bugfix

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.