OR filter group not respected

Problem/Motivation

Adding the filter inside an OR filter group doesn't put the condition inside an OR condition group.

Steps to reproduce

1. Add (normal) reference filter with operator is empty
2. Add entityreference filter (same field from above)
3. Create filter group with OR operator and add the 2 filters inside the filter group

Proposed resolution

ManyToOneHelper's "Reduce duplicates" will create the correct query so if we
show "Reduce duplicates" field in the value form/build options form, we can set that in the filter and it will fix the filter group.

Calling $this->helper->buildOptionsForm($form, $form_state); inside EntityReferenceFilterViewResult::valueForm() will do the trick