Fail to get schema ledger metadata for String schema

**Describe the bug**
If get topic internal stats with String schema, will throw NPE when getting ledger metadata by bookkeeper client. https://github.com/apache/pulsar/pull/10957#pullrequestreview-686291742 can fixes the NPE to cause get internal stats failed, but we also need a fix on the Pulsar side.

The issue names no source file, test, or entry point. Start by tracing the internal-stats path for String-schema topics and the ledger-metadata lookup described here, using the linked pull request review for context; done means internal stats succeeds without the reported NPE.

Written by the indexing model from the issue text.