Fix exclude-only multi filters matching nothing

MultiCriterion.to_q (used by SessionFilter for game/device) unconditionally added
field__in=value even when value was empty, and __in=[] matches no rows — so a
filter with only excludes (e.g. device excludes 11, no game/device includes)
returned zero results. Guard the empty value like ChoiceCriterion already does,
so an exclude-only criterion means 'all rows except the excluded ids'.

https://claude.ai/code/session_01XzhXvMvw42CQGc9kmin3GS

tests/test_filters.py

@@ -10,6 +10,7 @@ from common.criteria import (
     ChoiceCriterion,
     IntCriterion,
     Modifier,
+    MultiCriterion,
     StringCriterion,
 )
 from common.components import FilterBar
@@ -98,6 +99,29 @@ class TestChoiceCriterion:
         assert c.to_q("status") == ~Q(status__in=["f"])
 
 
+class TestMultiCriterion:
+    def test_includes(self):
+        c = MultiCriterion(value=[797], modifier=Modifier.INCLUDES)
+        assert c.to_q("game_id") == Q(game_id__in=[797])
+
+    def test_excludes_only_empty_value(self):
+        """Exclude one device with no includes — value=[], excludes=[11].
+
+        Regression: an empty ``value`` must not add ``__in=[]`` (which matches
+        nothing); the criterion should mean "all rows except device 11".
+        """
+        c = MultiCriterion(value=[], excludes=[11], modifier=Modifier.INCLUDES)
+        assert c.to_q("device_id") == ~Q(device_id__in=[11])
+
+    def test_include_and_exclude(self):
+        c = MultiCriterion(value=[1], excludes=[2], modifier=Modifier.INCLUDES)
+        assert c.to_q("game_id") == Q(game_id__in=[1]) & ~Q(game_id__in=[2])
+
+    def test_is_null(self):
+        c = MultiCriterion(value=[], modifier=Modifier.IS_NULL)
+        assert c.to_q("device_id") == Q(device_id__isnull=True)
+
+
 class TestChoiceCriterionAgainstDB:
     """Verify ChoiceCriterion produces correct DB results."""