[4.1.x] Fixed #33996 -- Fixed CheckConstraint validation on NULL values.

Bug in 667105877e.

Thanks James Beith for the report.

Backport of e14d08cd89 from main

django/db/backends/base/features.py

@@ -298,6 +298,9 @@ class BaseDatabaseFeatures:
     # Does the backend support boolean expressions in SELECT and GROUP BY
     # clauses?
     supports_boolean_expr_in_select_clause = True
+    # Does the backend support comparing boolean expressions in WHERE clauses?
+    # Eg: WHERE (price > 0) IS NOT NULL
+    supports_comparing_boolean_expr = True
 
     # Does the backend support JSONField?
     supports_json_field = True

django/db/backends/oracle/features.py

@@ -64,6 +64,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     supports_slicing_ordering_in_compound = True
     allows_multiple_constraints_on_same_fields = False
     supports_boolean_expr_in_select_clause = False
+    supports_comparing_boolean_expr = False
     supports_primitives_in_json_field = False
     supports_json_field_contains = False
     supports_collation_on_textfield = False

django/db/models/query_utils.py

@@ -12,7 +12,7 @@ import logging
 from collections import namedtuple
 
 from django.core.exceptions import FieldError
-from django.db import DEFAULT_DB_ALIAS, DatabaseError
+from django.db import DEFAULT_DB_ALIAS, DatabaseError, connections
 from django.db.models.constants import LOOKUP_SEP
 from django.utils import tree
 
@@ -120,7 +120,8 @@ class Q(tree.Node):
         matches against the expressions.
         """
         # Avoid circular imports.
-        from django.db.models import Value
+        from django.db.models import BooleanField, Value
+        from django.db.models.functions import Coalesce
         from django.db.models.sql import Query
         from django.db.models.sql.constants import SINGLE
 
@@ -131,7 +132,10 @@ class Q(tree.Node):
             query.add_annotation(value, name, select=False)
         query.add_annotation(Value(1), "_check")
         # This will raise a FieldError if a field is missing in "against".
-        query.add_q(self)
+        if connections[using].features.supports_comparing_boolean_expr:
+            query.add_q(Q(Coalesce(self, True, output_field=BooleanField())))
+        else:
+            query.add_q(self)
         compiler = query.get_compiler(using=using)
         try:
             return compiler.execute_sql(SINGLE) is not None