fix(api): require subname for PUT on RRsetDetail (as documented)

Previously, the serializer would fall back to the default empty subname.
On non-apex records, this resulted in an attempt to change the subname
value, which previously was rejected (with a different error). For apex
names, the operation would continue (because the default subname would
mean no change).

api/desecapi/serializers/records.py

@@ -81,7 +81,8 @@ def __init__(self, default):
 
     def __call__(self, serializer_field):
         is_many = getattr(serializer_field.root, "many", False)
-        if is_many:
+        partial = getattr(serializer_field.root, "partial", False)
+        if is_many or (serializer_field.root.instance and not partial):
             serializer_field.fail("required")
         if callable(self.default):
             if getattr(self.default, "requires_context", False):

api/desecapi/tests/test_rrsets.py

@@ -1236,6 +1236,22 @@ def test_update_my_rr_sets(self):
             )
             self.assertStatus(response, status.HTTP_400_BAD_REQUEST)
 
+    def test_update_my_rr_sets_missing_subname(self):
+        for subname in ["", "test"]:
+            with self.assertNoRequestsBut():
+                data = {
+                    "records": ["127.0.0.1"],
+                    "ttl": 3630,
+                    "type": "A",
+                }
+                self.assertBadRequest(
+                    self.client.put_rr_set(
+                        self.my_rr_set_domain.name, subname, "A", data
+                    ),
+                    "This field is required.",
+                    ("subname", 0),
+                )
+
     def test_update_my_rr_sets_wrong_subname(self):
         for s1, s2 in [("", "test"), ("test", "")]:
             with self.assertNoRequestsBut():