GCP: Fix listNamespaces to return empty list for existing namespaces

**Apache Iceberg version**
main @ 035fc1e40

**Query engine**
N/A — catalog-level bug, engine-agnostic

**Please describe the bug**
`BigQueryMetastoreCatalog#listNamespaces(Namespace)` always throws `NoSuchNamespaceException` for any non-empty namespace argument, even when the namespace exists (bigquery/src/main/java/org/apache/iceberg/gcp/bigquery/BigQueryMetastoreCatalog.java line 233-238, prior to fix).

This contradicts the method's own Javadoc: "Since this catalog only supports one-level namespaces, it always returns an empty list unless passed an empty namespace to list all namespaces within the catalog."

`HiveCatalog#listNamespaces(Namespace)` (hive-metastore/src/main/java/org/apache/iceberg/hive/HiveCatalog.java line 530-537) implements the correct pattern: check `namespaceExists(namespace)` first, return `ImmutableList.of()` if it exists, throw only if it does not.

**Steps to reproduce**
1. `catalog.createNamespace(Namespace.of("existing_ns"))`
2. `catalog.listNamespaces(Namespace.of("existing_ns"))`

Expected: empty list. Actual: `NoSuchNamespaceException`.

**Additional context**
N/A — covered above.

Start in bigquery/src/main/java/org/apache/iceberg/gcp/bigquery/BigQueryMetastoreCatalog.java at lines 233-238, then compare HiveCatalog#listNamespaces in hive-metastore/src/main/java/org/apache/iceberg/hive/HiveCatalog.java. Reproduce the listed createNamespace and listNamespaces calls; done means an existing non-empty namespace returns an empty list while a missing namespace still raises NoSuchNamespaceException.

Written by the indexing model from the issue text.