Fix validation of empty column names for compact tables

patch by slebresne; reviewed by jbellis for CASSANDRA-6152
Merge branch 'cassandra-1.2' into cassandra-2.0

Conflicts:
	src/java/org/apache/cassandra/cql3/QueryProcessor.java
	src/java/org/apache/cassandra/cql3/UpdateParameters.java
Merge branch 'cassandra-2.0' into trunk