Add empty migration to appease the CI backcompat test

Created by: efritz

I recently introduced two migrations that (1) added a non-nullable column, and (2) removed a column. Apparently this causes the backcompat tests in CI to throw a pretty severe (yet flaky) tantrum.

This PR tests to see if we add a migration file that does not do one of those actions if the test state becomes sane again.

See https://github.com/sourcegraph/sourcegraph/issues/10498 and https://sourcegraph.slack.com/archives/C07KZF47K/p1588961043191600 for additional context.