Make an empty DiffSync instance evaluate to True.

diffsync/__init__.py

@@ -456,6 +456,13 @@ def __str__(self):
     def __repr__(self):
         return f"<{str(self)}>"
 
+    def __bool__(self):
+        """Always evaluate DiffSync instances as True.
+
+        This is needed because without it the __len__ method would be used, which in turn would cause empty DiffSync
+        instances to evaluate as False."""
+        return True
+
     def __len__(self):
         """Total number of elements stored."""
         return self.store.count()

tests/unit/test_diffsync.py

@@ -894,3 +894,7 @@ class NoDeleteInterfaceDiffSync(BackendA):
     diff = extra_models.diff_from(backend_a)
     print(diff.str())  # for debugging of any failure
     assert not diff.has_diffs()
+
+
+def test_diffsync_empty_instance_is_truthy():
+    assert bool(DiffSync())