CDRIVER-5515 fix assert when legacy exhaust cursor receives no documents

[kevinAlbs]

Summary

Fixes a failed assert when an exhaust cursor receives no documents:

src/libbson/src/bson/bson-reader.c:534 bson_reader_new_from_data(): precondition failed: data

Changes verified with a patch build testing all server 3.6 tasks. 3.6 uses legacy exhaust cursors.

Analysis

Regression was introduced in 6f58b9d.

On the prior commit, the call to bson_reader_new_from_data was the following:

   response->reader =
      bson_reader_new_from_data (response->rpc.reply.documents,
                                 (size_t) response->rpc.reply.documents_len);

response->rpc.reply.documents was always non-NULL. When no documents are received, the pointer is assigned the address of the end of the OP_REPLY.

6f58b9d changed to use mcd_rpc_op_reply_get_documents:

   response->reader = bson_reader_new_from_data (
      mcd_rpc_op_reply_get_documents (response->rpc),
      mcd_rpc_op_reply_get_documents_len (response->rpc));

mcd_rpc_op_reply_get_documents returns NULL if there are no documents. This causes the assert failure in bson_reader_new_from_data.

This PR does not change behavior of mcd_rpc_op_reply_get_documents. Returning NULL is documented behavior. Though mcd_rpc_op_reply_get_documents is not in a public header, it is expected to be used by the Serverless Proxy (as part of CDRIVER-4625).