[PDI-11078]Fix malformed CQL batch generation

'INSERT INTO' statement may be incorrect if values before the key are null.
Fixed by reducing number of code paths and using standard Guava utilities.

http://jira.pentaho.com/browse/PDI-11078
Merge pull request #39 from hudak/PDI-11078_5.1

 [PDI-11078] Fix malformed CQL batch generation