Failing test on delete_all if no where clause is specified

Hello! Again, thanks for the great work! Playing with this in a medium sized project I stumbled against a weird problem: when calling Repo.delete_all with a query that is missing any other clause execpt the select, this fails because the nil in the {count, nil} result is being enumerated.

Not knowing well the internals of Ecto/Repo I managed only to go this far for now and will retry tomorrow, but it may be simpler than I thought, so for the moment I cloned this repo and created a failing example test to reproduce it here

  1) test Ecto.Adapter.Queryable#execute #delete_all from one table with select/only query, records (Ecto.Adapters.MnesiaQueryableIntegrationTest)
     test/ecto/adapters/mnesia/queryable_test.exs:455
     ** (Protocol.UndefinedError) protocol Enumerable not implemented for nil of type Atom. This protocol is implemented for the following type(s): Qlc.Cursor, MapSet, Date.Range, GenEvent.Stream, Stream, File.Stream, IO.Stream, Map, HashSet, List, Range, HashDict, Function
     code: case TestRepo.delete_all(from(s in TestSchema, select: s)) do
     stacktrace:
       (elixir) lib/enum.ex:1: Enumerable.impl_for!/1
       (elixir) lib/enum.ex:141: Enumerable.reduce/3
       (elixir) lib/enum.ex:3023: Enum.map/2
       (ecto) lib/ecto/repo/queryable.ex:234: Ecto.Repo.Queryable.execute/4
       test/ecto/adapters/mnesia/queryable_test.exs:469: (test)

note: editing mnesia.ex's execute/5 in line 240 returning {length(result), []} instead of {length(result), nil} allows delete_all to return successfully, but ofc existing tests relying on nil to be returned fail. From Ecto.Repo delete_all/1 docs

It returns a tuple containing the number of entries and any returned result as second element. The second element is nil by default unless a select is supplied in the delete query.

so it is unclear to me if [] or nil is the correct result, but you may have more information/sight than me!

Thanks again, let me know if I can add something or do something!