[1.10.x] Refs #24227 -- Fixed crash of ManyToManyField.value_from_object() on...

[1.10.x] Refs #24227 -- Fixed crash of ManyToManyField.value_from_object() on unsaved model instances.

This behavior was removed in 67d98441
but is needed to prevent a crash in formtools.

Backport of a4c20ae8 from master