Issue #17901: fix TreeBuilder construction for an explicit element_factory=None

Based on report and patch by Aaron Oakley.

Lib/test/test_xml_etree.py

@@ -1896,6 +1896,12 @@ class TreeBuilderTest(unittest.TestCase):
         parser.feed(self.sample1)
         self.assertIsNone(parser.close())
 
+    def test_treebuilder_elementfactory_none(self):
+        parser = ET.XMLParser(target=ET.TreeBuilder(element_factory=None))
+        parser.feed(self.sample1)
+        e = parser.close()
+        self._check_sample1_element(e)
+
     def test_subclass(self):
         class MyTreeBuilder(ET.TreeBuilder):
             def foobar(self, x):

Misc/ACKS

@@ -905,6 +905,7 @@ John O'Connor
 Kevin O'Connor
 Tim O'Malley
 Zooko O'Whielacronx
+Aaron Oakley
 James Oakley
 Jon Oberheide
 Pascal Oberndoerfer

Modules/_elementtree.c

@@ -2341,7 +2341,7 @@ treebuilder_handle_start(TreeBuilderObject* self, PyObject* tag,
         self->data = NULL;
     }
 
-    if (self->element_factory) {
+    if (self->element_factory && self->element_factory != Py_None) {
         node = PyObject_CallFunction(self->element_factory, "OO", tag, attrib);
     } else {
         node = create_new_element(tag, attrib);