fix coverage for 'conset()' (#1704)

pydantic/types.py

@@ -190,9 +190,6 @@ class ConstrainedSet(set):  # type: ignore
 
     @classmethod
     def set_length_validator(cls, v: 'Optional[Set[T]]', field: 'ModelField') -> 'Optional[Set[T]]':
-        if v is None and not field.required:
-            return None
-
         v = set_validator(v)
         v_len = len(v)
 

tests/test_types.py

@@ -405,6 +405,14 @@ def test_conset():
     assert exc_info.value.errors() == [{'loc': ('foo',), 'msg': 'value is not a valid set', 'type': 'type_error.set'}]
 
 
+def test_conset_not_required():
+    class Model(BaseModel):
+        foo: Set[int] = None
+
+    assert Model(foo=None).foo is None
+    assert Model().foo is None
+
+
 class ConStringModel(BaseModel):
     v: constr(max_length=10) = 'foobar'