[5.0.x] Fixed #35594 -- Added unique nulls distinct validation for expressions.

Thanks Mark Gensler for the report.

Backport of adc0b6aac3 from main.

django/db/models/constraints.py

@@ -6,7 +6,7 @@ from django.core.exceptions import FieldError, ValidationError
 from django.db import connections
 from django.db.models.expressions import Exists, ExpressionList, F, OrderBy
 from django.db.models.indexes import IndexExpression
-from django.db.models.lookups import Exact
+from django.db.models.lookups import Exact, IsNull
 from django.db.models.query_utils import Q
 from django.db.models.sql.query import Query
 from django.db.utils import DEFAULT_DB_ALIAS
@@ -427,13 +427,17 @@ class UniqueConstraint(BaseConstraint):
                     meta=model._meta, exclude=exclude
                 ).items()
             }
-            expressions = []
+            filters = []
             for expr in self.expressions:
                 # Ignore ordering.
                 if isinstance(expr, OrderBy):
                     expr = expr.expression
-                expressions.append(Exact(expr, expr.replace_expressions(replacements)))
-            queryset = queryset.filter(*expressions)
+                rhs = expr.replace_expressions(replacements)
+                condition = Exact(expr, rhs)
+                if self.nulls_distinct is False:
+                    condition = Q(condition) | Q(IsNull(expr, True), IsNull(rhs, True))
+                filters.append(condition)
+            queryset = queryset.filter(*filters)
         model_class_pk = instance._get_pk_val(model._meta)
         if not instance._state.adding and model_class_pk is not None:
             queryset = queryset.exclude(pk=model_class_pk)