IMPALA-759: Parquet writer: all NULL columns should use PLAIN encoding.
IMPALA-759: Parquet writer: all NULL columns should use PLAIN encoding.

Change-Id: I35d5997d7623239f6cfce7f97a5802545126ea4c
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1764
Reviewed-by: Nong Li <99a5e5f8f5911755b88e0b536d46aafa102bed41@cloudera.com>
Tested-by: jenkins

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