Allow untagged unit variants to deserialize from Visitor::visit_none()

test_suite/tests/test_de.rs

@@ -137,6 +137,12 @@ enum EnumOther {
     Other,
 }
 
+#[derive(PartialEq, Debug, Deserialize)]
+#[serde(untagged)]
+enum EnumUntaggedUnit {
+    Unit,
+}
+
 #[derive(PartialEq, Debug)]
 struct IgnoredAny;
 
@@ -788,6 +794,9 @@ declare_tests! {
             Token::Unit,
         ],
     }
+    test_enum_untagged_unit_from_none {
+        UntaggedUnit::Unit => &[Token::None],
+    }
     test_box {
         Box::new(0i32) => &[Token::I32(0)],
     }