Fixes a segfault when querying for an empty keyspace.
Also, fixes an infinite loop on smp > 1. Queries to
system.size_estimates table which are not single-partition queries
caused Scylla to go into an infinite loop inside
multishard_combining_reader::fill_buffer. This happened because
multishard_combinind_reader assumes that shards return rows belonging
to separate partitions, which was not the case for
size_estimates_mutation_reader.
Fixes#4689.
14700c2ac4