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

common/criteria.py

@@ -277,7 +277,9 @@ class MultiCriterion(_Criterion):
     def to_q(self, field_name: str) -> Q:
         m = self.modifier
         if m == Modifier.INCLUDES:
-            q = Q(**{f"{field_name}__in": self.value})
+            q = Q()
+            if self.value:
+                q &= Q(**{f"{field_name}__in": self.value})
             if self.excludes:
                 q &= ~Q(**{f"{field_name}__in": self.excludes})
             return q