test: don't disable topology test_null test

With the fix in `boot_strapper` from the previous commit, bootstrapping
clusters with the topology harness no longer fails with `no_such_keyspace`
error. We can enable the first test.

Author: kbr-scylla

test/topology/suite.yaml

@@ -3,5 +3,4 @@ pool_size: 4
 topology:
     class: Simple
     replication_factor: 3
-disable:
-    - test_null
+disable: []