When a branch is created, there is no previous commit to reference in the `github.event.before` context, which causes the performance workflow to fail because there is no previous commit to perform a comparison with.
This adds a condition to check that `github.event.before` is not set to `0000000000000000000000000000000000000000`, which is the default value when there are no previous commits.
Props swissspidy.
See #588867.
git-svn-id: https://develop.svn.wordpress.org/trunk@56972 602fd350-edb4-49c9-b593-d223f7449a82