Unblock test `listNamespacesWithEmptyNamespace`

[liamzwbao]

Describe the bug

The test listNamespacesWithEmptyNamespace in IcebergCatalogTest is disabled in #1126 as the expected behavior is not applicable to Polaris.

To unblock, do one of the following:

1. Align Polaris behavior with the superclass by handling empty namespaces the same way, or
2. Modify this test to expect an exception and add a Polaris-specific version.

To Reproduce

Remove the @Disabled annotation and run the test.

Actual Behavior

The test will fail because Polaris implicitly creates an empty namespace, whereas the test expects the empty namespace to not exist unless the user explicitly creates it.

Expected Behavior

TBD, needs discussion.

Additional context

No response

System information

No response

[flyrain]

The test will fail because Polaris implicitly creates an empty namespace, whereas the test expects the empty namespace to not exist unless the user explicitly creates it.

I think we should figure out the reason of "Polaris implicitly creates an empty namespace" first. cc @eric-maynard @dennishuo @collado-mike for more context.

[liamzwbao]

The ROOT namespace is added here. I guess this is necessary because every created entity requires a parent, and the ROOT namespace serves as the parent for all other namespaces under the root.

One thing to note is that it's currently possible to drop the ROOT namespace. If a user does so, they will no longer be able to create any new namespaces in Polaris.