Use False instead of None for test values

bookwyrm/models/status.py

@@ -376,7 +376,8 @@ class Review(BookStatus):
 
     def save(self, *args, **kwargs):
         """clear rating caches"""
-        cache.delete(f"book-rating-{self.book.parent_work.id}-*")
+        if self.book.parent_work:
+            cache.delete(f"book-rating-{self.book.parent_work.id}-*")
         super().save(*args, **kwargs)