fix(peewee): return None from ajax get_one() when no record matches

[DSeaStar]

Summary

QueryAjaxModelLoader.get_one() in the peewee backend called self.model.get(...), which raises DoesNotExist when no row matches. AjaxSelectField._get_data already guards with if model is not None, so a form that still holds a deleted (or otherwise unmatched) pk crashed instead of rendering with no selection.

The sqla and mongoengine loaders already return None on a miss, and the same peewee pattern was fixed for ModelView.get_one() in #2939. This applies that change to the ajax loader left out of that PR.

def get_one(self, pk):
    try:
        return self.model.get(**{self.pk: pk})
    except DoesNotExist:
        return None

Closes #2940.

Test plan

• test_ajax_fk now asserts loader.get_one(999999) is None
• Creating a form and feeding a missing pk no longer raises; the field data stays None
• Full peewee suite: pytest flask_admin/tests/peeweemodel/ (13 passed)

[ElLorans]

Why do we need this part of the test?