Fixed #34544 -- Avoided DBMS_LOB.SUBSTR() wrapping with IS NULL condition on Oracle.

Regression in 09ffc5c121.

Thanks Michael Smith for the report.

This also reverts commit 1e4da43955.

tests/lookup/models.py

@@ -19,6 +19,7 @@ class Alarm(models.Model):
 class Author(models.Model):
     name = models.CharField(max_length=100)
     alias = models.CharField(max_length=50, null=True, blank=True)
+    bio = models.TextField(null=True)
 
     class Meta:
         ordering = ("name",)
@@ -50,22 +51,11 @@ class NulledTextField(models.TextField):
         return None if value == "" else value
 
 
-class NullField(models.Field):
-    pass
-
-
-NullField.register_lookup(IsNull)
-
-
 @NulledTextField.register_lookup
 class NulledTransform(models.Transform):
     lookup_name = "nulled"
     template = "NULL"
 
-    @property
-    def output_field(self):
-        return NullField()
-
 
 @NulledTextField.register_lookup
 class IsNullWithNoneAsRHS(IsNull):

tests/lookup/tests.py

@@ -49,7 +49,7 @@ class LookupTests(TestCase):
     @classmethod
     def setUpTestData(cls):
         # Create a few Authors.
-        cls.au1 = Author.objects.create(name="Author 1", alias="a1")
+        cls.au1 = Author.objects.create(name="Author 1", alias="a1", bio="x" * 4001)
         cls.au2 = Author.objects.create(name="Author 2", alias="a2")
         # Create a few Articles.
         cls.a1 = Article.objects.create(
@@ -1029,6 +1029,13 @@ class LookupTests(TestCase):
         Season.objects.create(year=2012, gt=None)
         self.assertQuerySetEqual(Season.objects.filter(gt__regex=r"^$"), [])
 
+    def test_textfield_exact_null(self):
+        with self.assertNumQueries(1) as ctx:
+            self.assertSequenceEqual(Author.objects.filter(bio=None), [self.au2])
+        # Columns with IS NULL condition are not wrapped (except PostgreSQL).
+        bio_column = connection.ops.quote_name(Author._meta.get_field("bio").column)
+        self.assertIn(f"{bio_column} IS NULL", ctx.captured_queries[0]["sql"])
+
     def test_regex_non_string(self):
         """
         A regex lookup does not fail on non-string fields