Fixed #25772 -- Corrected __len lookup on ArrayField for empty arrays.

django/contrib/postgres/fields/array.py

@@ -202,7 +202,11 @@ class ArrayLenTransform(Transform):
 
     def as_sql(self, compiler, connection):
         lhs, params = compiler.compile(self.lhs)
-        return 'array_length(%s, 1)' % lhs, params
+        # Distinguish NULL and empty arrays
+        return (
+            'CASE WHEN %(lhs)s IS NULL THEN NULL ELSE '
+            'coalesce(array_length(%(lhs)s, 1), 0) END'
+        ) % {'lhs': lhs}, params
 
 
 class IndexTransform(Transform):