empty namespace [] in rest catalog

[subkanthi]

We need to identify how the empty namespace [] was created and implement a functionality to delete the namespace []

Debug> RestCatalog(s3://stag-01): Requesting: http:/v1/namespaces
<Debug> RestCatalog(s3://stag-01): Received response: {"namespaces":[[""],["iceberg"]],"next-page-token":null}
<Debug> RestCatalog(s3://stag-01): Loaded 2 namespaces in base namespace 
<Debug> RestCatalog(s3://stag-01): Requesting: http://namespaces

[xieandrew]

This can happen using: ice create-namespace -p '.test', which will create both an empty namespace and test.

The empty namespace can be deleted using ice delete-namespace '.test'

Edit: This doesn't work on etcd catalog. See #107 for the fix.

We should add error handling to prevent create-namespace from accepting empty namespace levels.