Fix Assertion errors when using empty clustering with static columns

patch by slebresne; reviewed by iamaleksey for CASSANDRA-7455
Merge branch 'cassandra-2.0' into cassandra-2.1.0

Conflicts:
	src/java/org/apache/cassandra/cql3/statements/ColumnGroupMap.java
Merge branch 'cassandra-2.1.0' into cassandra-2.1
Merge branch 'cassandra-2.1' into trunk