storage: do not fail __repr__ for criterias without value (#57914)

tests/test_misc.py

@@ -482,6 +482,9 @@ def test_criteria_repr():
     assert 'foo' in repr(criteria)
     assert 'bar' in repr(criteria)
 
+    criteria = wcs.qommon.storage.Null('foo')
+    assert 'Null' in repr(criteria)
+
 
 def test_related_field_repr():
     from wcs.backoffice.management import RelatedField

wcs/qommon/storage.py

@@ -158,7 +158,11 @@ class Criteria:
         return lambda x: self.op(getattr(x, self.attribute, None) or self.typed_none, self.value)
 
     def __repr__(self):
-        return '<%s (attribute: %r, value: %r)>' % (self.__class__.__name__, self.attribute, self.value)
+        return '<%s (attribute: %r%s)>' % (
+            self.__class__.__name__,
+            self.attribute,
+            ', value: %r' % self.value if hasattr(self, 'value') else '',
+        )
 
 
 class Less(Criteria):