EnumerableMergeJoin#create does not set EnumerableConvention in the trait set

EnumerableMergeJoin#create does not set EnumerableConvention in the trait set that will be passed to EnumerableMergeJoin's constructor, so it effectively creates an EnumerableMergeJoin with Convention.NONE.

So far, this problem had been unnoticed because EnumerableMergeJoinRule calls directly EnumerableMergeJoin's constructor (and not EnumerableMergeJoin#create).