Swen Kooij e6527fff4c Don't double key transform in lookups
In Django 4.2, `process_lhs()` is called for IsNull lookups,
which is how this bug came to appear. It was already there
and could be reproduced on Django 3.2.

The issue is when `LocalizedRef` is combined with a localized
lookup. The lookup is unaware of the existing localized ref
and adds another transform on top. This results in the
expression becoming:

```
field->'en'->'en'
```
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