Update upload_confirmed_blocks() return value when no blocks to upload (#33861)

upload_confirmed_blocks() states that it will return the passed in
ending_slot when there are no blocks to upload. This is enforced in one
early return but not the other. The result is that BigTableUploadService
could potentially get stuck in a loop of trying to upload the same slot.

While this case seems to be caused when an operator restarts their node
without --no-snapshot-fetch (which can cause a gap in blockstore), we
can still be friendly and allow them to break out of this loop.

ledger/src/bigtable_upload.rs

@@ -138,7 +138,7 @@ pub async fn upload_confirmed_blocks(
             "No blocks between {} and {} need to be uploaded to bigtable",
             starting_slot, ending_slot
         );
-        return Ok(last_blockstore_slot);
+        return Ok(ending_slot);
     }
     let last_slot = *blocks_to_upload.last().unwrap();
     info!(