[11.x] Fix flat array parameter for relation upsert

[iamgergo]

This is a followup of #52239.

As is turns it's possible to pass a flat array to the upsert method, not only nested arrays.

This PR makes sure using the upsert method with flat arrays on a relation, works.

I used the ! is_array(reset($values)) check for nested arrays, because the query builder's upsert method does the same.

Also updated the tests.