[1.5.x] Made custom m2m fields without through easier to use

The change in f105fbe52b21da206bfbaedf0e92326667d7b2d4 made through=None
m2m fields fail in cases where they worked before. It isn't possible to
create such fields using public APIs. The fix is trivial, so it seems
worth adding it.

This is not a backport from master. Master has gotten enough other
changes to related fields internal API that this fix alone isn't enough
to do any good.