"push with authfile" integration test: don't try to push an empty list

When testing that we can push a manifest list, don't try to push an
empty list, which now triggers an error in the libraries.

Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>

Author: nalind

tests/push.bats

@@ -200,7 +200,7 @@ load helpers
   run_buildah push --authfile ${TEST_SCRATCH_DIR}/tmp/test.auth $WITH_POLICY_JSON --tls-verify=false busybox docker://localhost:${REGISTRY_PORT}/buildah/busybox:latest
   expect_output --substring "Copying"
 
-  run_buildah manifest create localhost:${REGISTRY_PORT}/testmanifest
+  run_buildah manifest create localhost:${REGISTRY_PORT}/testmanifest busybox
   run_buildah manifest push --authfile ${TEST_SCRATCH_DIR}/tmp/test.auth $WITH_POLICY_JSON --tls-verify=false localhost:${REGISTRY_PORT}/testmanifest
   expect_output --substring "Writing manifest list to image destination"
 }