Fixed #22421 -- Regression in fixtures loading.

Loading fixtures were failing since the refactoring in 244e2b71f5 for
inheritance setups where the chain contains abstract models and the
root ancestor contains a M2M relation.

Thanks Stanislas Guerra for the report.

Refs #20946.

tests/fixtures_regress/fixtures/feature.json

@@ -0,0 +1,17 @@
+[
+{
+  "fields": {
+    "channels": [],
+    "title": "Title of this feature article"
+  },
+  "model": "fixtures_regress.article",
+  "pk": 1
+},
+{
+  "fields": {
+    "channels": []
+  },
+  "model": "fixtures_regress.feature",
+  "pk": 1
+}
+]