Raise when primary keys in Ecto are nils

[szymonkozak]

In some cases, when we have an Ecto payload, primary key can be set to nil. This can lead to situation where dataloader is associating the incorrect child object to a parent object. (See the issue here: #172).

This helps to avoid overlooking such situations by raising when nil was found in primary keys.

[szymonkozak]

Alternative solution would be to use

 record
    |> :erlang.term_to_iovec()
    |> :erlang.md5()

when we find nils in primary keys (so the same situation like without primary key at all). Then it should work even without primary keys loaded. But I didn't test it and I don't know if it's better. @benwilson512 wdyt?