Pluck on NullRelation accepts a list of columns

`pluck` was updated to accept a list of columns, but the `NullRelation`
was never updated to match that signature. As a result, calling `pluck`
on a `NullRelation` results in an `ArgumentError`.
Merge pull request #12547 from derekprior/dp-fix-null-relation-pluck

Pluck on NullRelation should accept a list of columns