Ecto.Validator should handle correct validations properly

Adding the test below to Ecto.ValidatorTest causes the following failure:

```elixir
  1) test struct dispatches to also (Ecto.ValidatorTest)
     test/ecto/validator_test.exs:66
     ** (FunctionClauseError) no function clause matching in :maps.fold/3
     stacktrace:
       (stdlib) maps.erl:171: :maps.fold(#Function<1.9776403/3 in Map.merge/3>, %{}, nil)
       test/ecto/validator_test.exs:74
```

Adding a default value in the `process_each/4` function of `lib/ecto/validator.ex` fixes the test, though I'm not sure if it fixes the underlying issue.