Allow None values in non required LTI fields.

We have seen at least one school sending `null` here which caused the
validation to fail for a field that was not required at all.

lms/validation/_lti_launch_params.py

@@ -26,10 +26,10 @@ class Meta:
     user_id = fields.Str(required=True)
     roles = fields.Str(required=True)
     tool_consumer_instance_guid = fields.Str(required=True)
-    lis_person_name_given = fields.Str(load_default="")
-    lis_person_name_family = fields.Str(load_default="")
-    lis_person_name_full = fields.Str(load_default="")
-    lis_person_contact_email_primary = fields.Str(load_default="")
+    lis_person_name_given = fields.Str(load_default="", allow_none=True)
+    lis_person_name_family = fields.Str(load_default="", allow_none=True)
+    lis_person_name_full = fields.Str(load_default="", allow_none=True)
+    lis_person_contact_email_primary = fields.Str(load_default="", allow_none=True)
 
     @pre_load
     def _decode_jwt(self, data, **_kwargs):

tests/unit/lms/validation/_lti_launch_params_test.py

@@ -126,6 +126,22 @@ def test_it_raises_LTIToolRedirect_if_a_reportable_field_is_missing(
             == f"http://example.com?lti_msg=Field+%27{required_param}%27%3A+Missing+data+for+required+field."
         )
 
+    @pytest.mark.parametrize(
+        "required_param",
+        [
+            "lis_person_name_given",
+            "lis_person_name_family",
+            "lis_person_name_full",
+            "lis_person_contact_email_primary",
+        ],
+    )
+    def test_it_allow_none_values(self, pyramid_request, required_param):
+        pyramid_request.params[required_param] = None
+
+        schema = BasicLTILaunchSchema(pyramid_request)
+
+        schema.parse()
+
     @pytest.mark.parametrize(
         "required_param",
         [