Add test for the fix.

Author: rwgk

tests/test_pytypes.cpp

@@ -289,6 +289,11 @@ TEST_SUBMODULE(pytypes, m) {
         return capsule;
     });
 
+    m.def("return_capsule_with_explicit_nullptr_dtor", []() {
+        py::print("creating capsule with explicit nullptr dtor");
+        return py::capsule((void *) 1234, (void (*)(void *)) nullptr); // PR #4221
+    });
+
     // test_accessors
     m.def("accessor_api", [](const py::object &o) {
         auto d = py::dict();

tests/test_pytypes.py

@@ -299,6 +299,17 @@ def test_capsule(capture):
     """
     )
 
+    with capture:
+        a = m.return_capsule_with_explicit_nullptr_dtor()
+        del a
+        pytest.gc_collect()
+    assert (
+        capture.unordered
+        == """
+        creating capsule with explicit nullptr dtor
+    """
+    )
+
 
 def test_accessors():
     class SubTestObject: