Tests for PR from fork are not working

[Mantisus]

With the current configuration of Github Actions, tests are not running with error 'CODECOV_TOKEN is missing'.

https://github.com/apify/apify-sdk-python/actions/runs/20719316730/job/59478494234?pr=727

[vdusek]

Let's try to skip the codecov upload if codecov token is not available -> apify/workflows#240.