[1.7.x] Fixed #24236 -- Treated inherited m2m fields as such if they don't...

[1.7.x] Fixed #24236 -- Treated inherited m2m fields as such if they don't define get_internal_type()

Regression introduced in 3d4a8261

Thanks IRC user ris for the report, Loïc Bistuer, Anssi Kääriäinen and
Andriy Sokolovskiy for the discussion and Tim Graham for the review.