Refactor nil map joins

[greg-rychlewski]

I think this is cleaner and more general than my last PR. But wanted to get your opinion.

In terms of more general, with this PR we not only handle merges but also the select {map(...from..), map(...join...)} situation we were talking about.

I'm not sure how obvious it is why this works. But basically here:

defp preprocessor({_, {:source, {source, schema}, prefix, types}}, preprocess, adapter) do
    struct = Ecto.Schema.Loader.load_struct(schema, prefix, source)

    fn row ->
      {entry, rest} = struct_load!(types, row, [], false, struct, adapter)
      preprocess(rest, preprocess, entry, adapter)
    end
  end

If we are using map/2 then schema is converted to nil in the planner and then struct is given the value %{}.