feat: XmlSerializer skip optional nillable elements

tests/fixtures/models.py

@@ -172,6 +172,8 @@ class SequentialType:
     x4: Optional[int] = field(
         default=None, metadata={"type": "Element", "sequence": 2}
     )
+    x5: Optional[str] = field(default=None, metadata={"type": "Element", "nillable": True})
+    x6: Optional[str] = field(default=None, metadata={"type": "Element", "nillable": True, "required": True})
 
 
 @dataclass

tests/formats/dataclass/serializers/test_mixins.py

@@ -693,6 +693,10 @@ def test_convert_choice_with_derived_dataclass(self):
         expected = [
             ("start", "a"),
             ("attr", "a0", "foo"),
+            ("start", "{xsdata}x6"),
+            ("attr", QNames.XSI_NIL, "true"),
+            ("data", None),
+            ("end", "{xsdata}x6"),
             ("end", "a"),
         ]
 
@@ -814,6 +818,7 @@ def test_next_value(self):
         x2 = next(meta.find_children("x2"))
         x3 = next(meta.find_children("x3"))
         x4 = next(meta.find_children("x4"))
+        x6 = next(meta.find_children("x6"))
 
         actual = self.generator.next_value(obj, meta)
         expected = [
@@ -825,6 +830,7 @@ def test_next_value(self):
             (x1, 4),
             (x3, 9),
             (x4, 10),
+            (x6, None),
         ]
 
         self.assertIsInstance(actual, Generator)

xsdata/formats/dataclass/serializers/mixins.py

@@ -933,7 +933,7 @@ def next_value(cls, obj: Any, meta: XmlMeta) -> Iterator[Tuple[XmlVar, Any]]:
 
             if var.sequence is None:
                 value = getattr(obj, var.name)
-                if value is not None or var.nillable:
+                if value is not None or (var.nillable and var.required):
                     yield var, value
                 index += 1
                 continue