bpo-42773: fix tests not being run on pushes (GH-24004)

There was a typo, we were checking if the "GITHUB_BASE_REF" string
literal was empty instead of the $GITHUB_BASE_REF value. When
$GITHUB_BASE_REF is empty, the action that triggered the run was not a
pull request, so we always run the full test suite.

Signed-off-by: Filipe Laíns <lains@riseup.net>