Fix NPE on natural join

default> create table t(a int);
default> insert overwrite into t select 1;
default> create table s(a int);
default> insert overwrite into s select 1;
default> select * from t natural join s;

   public RESULT visitJoin(CONTEXT ctx, Stack<Expr> stack, Join expr) throws PlanningException {
     stack.push(expr);
-    visit(ctx, stack, expr.getQual());
+    if (expr.getQual() != null) {
+      visit(ctx, stack, expr.getQual());
+    }

-    join.joinQual = (Expr) joinQual.clone();
+    if (joinQual != null) {
+      join.joinQual = (Expr) joinQual.clone();
+    }

default> select * from t natural join s;
Progress: 100%, response time: 0.293 sec
a,  a
-------------------------------
1,  1
(1 rows, 0.293 sec, 4 B selected)