Summary:
The patch fixes a feature interaction bug between BlobDB and the `GetEntity` API:
without the patch, `GetEntity` would return the blob reference (wrapped into a
single-column entity) instead of the actual blob value.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11162

Test Plan: `make check`

Reviewed By: akankshamahajan15

Differential Revision: D42854092

Pulled By: ltamasi

fbshipit-source-id: f750d0ff57def107da16f545077ddce9860ff21a