sstableloader: Handle null-ish clustering values
sstableloader: Handle null-ish clustering values

Fixes #57 (second part)

It seems cassandra allows timestamp clustering columns to assume
null (or null-interpreted) values by "set col=''"

This causes the sstable reader to give us back null values for
the clustering prefix. Code assumed we where then done with
restriction, and omitted the value.

Fix by checking iff we still have more clustering to add,
and in that case do a restriction on null (which is accepted
by java driver and protocol, if not by cqlsh.

Message-Id: <b87b799212c86e6b55cdf03bf2363f79ff28d41c@scylladb.com>

scylladb/scylla-tools-java,scylladb/scylla-tools-java,scylladb/scylla-tools-java,scylladb/scylla-tools-java