cqlsh: fix COPY FROM handling of null/empty PK values

Patch by Tyler Hobbs; reviewed by Aleksey Yeschenko for CASSANDRA-7792
Merge branch 'cassandra-2.1.0' into cassandra-2.1
Merge branch 'cassandra-2.1' into trunk
(cqlsh) COPY TO/FROM improvements

patch by Mikhail Stepura; reviewed by Tyler Hobbs for CASSANDRA-7405
Merge branch 'cassandra-2.1' into trunk