test_provider_wfs.py: partial fixes for issues with Qt6

There are still 4 remaining failures on Qt6 builds, but that's a
start...

tests/src/python/test_provider_wfs.py

@@ -43,10 +43,13 @@
 )
 from qgis.PyQt.QtCore import (
     QCoreApplication,
+    QDate,
     QDateTime,
     QEventLoop,
     QObject,
-    Qt, QTime, QDate)
+    Qt,
+    QTime,
+)
 import unittest
 from qgis.testing import start_app, QgisTestCase
 from utilities import compareWkt, unitTestDataPath
@@ -4674,21 +4677,21 @@ def test_NullValues_regression_20961(self):
         self.assertTrue(vl.isValid())
 
         got_f = [f for f in vl.getFeatures()]
-        self.assertEqual(str(got_f[0]['type']), 'NULL')
-        self.assertEqual(str(got_f[0]['elevation']), 'NULL')
+        self.assertIsNone(got_f[0]['type'])
+        self.assertIsNone(got_f[0]['elevation'])
         self.assertEqual(str(got_f[0]['name']), 'Xxx')
         self.assertEqual(str(got_f[1]['type']), '0')
-        self.assertEqual(str(got_f[1]['elevation']), 'NULL')
+        self.assertIsNone(got_f[1]['elevation'])
         self.assertEqual(str(got_f[1]['name']), 'sdf')
 
         # Now iterate ! Regression #20961
         ids = [f.id() for f in got_f]
         got_f2 = [vl.getFeature(id) for id in ids]
-        self.assertEqual(str(got_f2[0]['type']), 'NULL')
-        self.assertEqual(str(got_f2[0]['elevation']), 'NULL')
+        self.assertIsNone(got_f2[0]['type'])
+        self.assertIsNone(got_f2[0]['elevation'])
         self.assertEqual(str(got_f2[0]['name']), 'Xxx')
         self.assertEqual(str(got_f2[1]['type']), '0')
-        self.assertEqual(str(got_f2[1]['elevation']), 'NULL')
+        self.assertIsNone(got_f2[1]['elevation'])
         self.assertEqual(str(got_f2[1]['name']), 'sdf')
 
     def testFilteredFeatureRequests(self):