roachtest: Monitor.Wait should work correctly without active tasks

[miretskiy]

PR #107548 made it so that
it is no longer safe to call Wait() method on a monitor when no tasks were started.

This is undesirable -- wait method should work correctly, as it did prior to the above PR

Jira issue: CRDB-30509

[blathers-crl]

cc @cockroachdb/test-eng