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'
```

localized_fields/lookups.py

@@ -23,6 +23,7 @@ from django.db.models.lookups import (
     StartsWith,
 )
 from django.utils import translation
+from psqlextra.expressions import HStoreColumn
 
 from .fields import LocalizedField
 
@@ -37,14 +38,33 @@ except ImportError:
 
 class LocalizedLookupMixin:
     def process_lhs(self, qn, connection):
-        if isinstance(self.lhs, Col):
+        # If the LHS is already a reference to a specific hstore key, there
+        # is nothing to be done since it already references as specific language.
+        if isinstance(self.lhs, HStoreColumn) or isinstance(
+            self.lhs, KeyTransform
+        ):
+            return super().process_lhs(qn, connection)
+
+        # If this is something custom expression, we don't really know how to
+        # handle that, so we better do nothing.
+        if not isinstance(self.lhs, Col):
+            return super().process_lhs(qn, connection)
+
+        # Select the key for the current language. We do this so that
+        #
+        # myfield__<lookup>=
+        #
+        # Is converted into:
+        #
+        # myfield__<lookup>__<current language>=
         language = translation.get_language() or settings.LANGUAGE_CODE
         self.lhs = KeyTransform(language, self.lhs)
+
         return super().process_lhs(qn, connection)
 
     def get_prep_lookup(self):
-        # Django 4.0 removed the ability for isnull fields to be something other than a bool
-        # We should NOT convert them to strings
+        # Django 4.0 removed the ability for isnull fields to be something
+        # other than a bool We should NOT convert them to strings
         if isinstance(self.rhs, bool):
             return self.rhs
         return str(self.rhs)

tests/test_lookups.py

@@ -3,6 +3,7 @@ from django.conf import settings
 from django.test import TestCase, override_settings
 from django.utils import translation
 
+from localized_fields.expressions import LocalizedRef
 from localized_fields.fields import LocalizedField
 from localized_fields.value import LocalizedValue
 
@@ -49,6 +50,18 @@ class LocalizedLookupsTestCase(TestCase):
         # ensure that hstore lookups still work
         assert self.TestModel.objects.filter(text__ro="text_ro").exists()
 
+    def test_localized_lookup_specific_isnull(self):
+        self.TestModel.objects.create(
+            text=LocalizedValue(dict(en="text_en", ro="text_ro", nl=None))
+        )
+
+        translation.activate("nl")
+        assert (
+            self.TestModel.objects.annotate(text_localized=LocalizedRef("text"))
+            .filter(text_localized__isnull=True)
+            .exists()
+        )
+
 
 class LocalizedRefLookupsTestCase(TestCase):
     """Tests whether ref lookups properly work with."""