Small fix for empty results of LAST_ENTRY_AT query

I caught a small edge case in the cake_query module: if the query LAST_ENTRY_AT is performed on  an empty file, an empty list [] is returned instead of an empty binary <<>>, which makes the byte_size() function on line 103 of cake_protocol crash. I made a small quick fix in this pull request, but:

Note that this issue is already fixed in @GARY46's  pull request https://github.com/pmembrey/cakedb/pull/16, so it might be preferable to merge  https://github.com/pmembrey/cakedb/pull/16 and close this pull request.