Fix explain(locus) issues.

[avamingli]

Fix typo and locus null as below.

explain (costs off, locus)
select * from dedup_reptab r where r.a in (select t.a/10 from dedup_tab t);
                                      QUERY PLAN
------------------------------------------------------------------------
 Gather Motion 3:1  (slice1; segments: 3)
   Locus: Entry
   ->  Result
         Locus: Strewn
         ->  Unique
               Locus: NULL
               Group Key: (RowIdExpr)
               ->  Sort
                     Locus: NULL
                     Sort Key (Distinct): (RowIdExpr)
                     ->  Redistribute Motion 3:3  (slice2; segments: 3)
                           Locus: Hashed
                           Hash Key: (RowIdExpr)
                           ->  Hash Join
                                 Locus: Hashed
                                 Hash Cond: ((t.a / 10) = r.a)
                                 ->  Seq Scan on dedup_tab t
                                       Locus: Hashed
                                 ->  Hash
                                       Locus: Replicated
                                       ->  Broadcast Motion 1:3
(slice3; segments: 1)
                                             Locus: Replicated
                                             ->  Seq Scan on
dedup_reptab r
                                                   Locus: SingleQE

Authored-by: Zhang Mingli [email protected]

fix #ISSUE_Number

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