Fixed #34779 -- Avoided unnecessary selection of non-nullable m2m fields without natural keys during serialization.

By using `select_related(None)` instead of `select_related()`, the
unnecessary joins are completely avoided. Note that the current tests
already covers the change, when the field is not `null=True`.

Regression in f9936deed1.

tests/serializers/models/base.py

@@ -60,7 +60,7 @@ class TopicManager(models.Manager):
 
 class Topic(models.Model):
     name = models.CharField(max_length=255)
-    category = models.ForeignKey(Category, models.CASCADE, null=True)
+    category = models.ForeignKey(Category, models.CASCADE)
     objects = TopicManager()