Filter not getting pushed down on MapR-DB tables.

0: jdbc:drill:zk=local> explain plan for select * from dfs.`/tmp/js` where b is null;
ANTLR Tool version 4.5 used for code generation does not match the current runtime version 4.7.1ANTLR Runtime version 4.5 used for parser compilation does not match the current runtime version 4.7.1ANTLR Tool version 4.5 used for code generation does not match the current runtime version 4.7.1ANTLR Runtime version 4.5 used for parser compilation does not match the current runtime version 4.7.1+----------------------------------------------------------------------------------+----------------------------------------------------------------------------------+
| text | json |
+----------------------------------------------------------------------------------+----------------------------------------------------------------------------------+
| 00-00 Screen
00-01 Project(**=[$0])
00-02 Project(T0¦¦**=[$0])
00-03 SelectionVectorRemover
00-04 Filter(condition=[IS NULL($1)])
00-05 Project(T0¦¦**=[$0], b=[$1])
00-06 Scan(table=[[dfs, /tmp/js]], groupscan=[JsonTableGroupScan [ScanSpec=JsonScanSpec [tableName=/tmp/js, condition=null], columns=[`**`, `b`], maxwidth=1]])