chore: test and handle the case of self being a null-valued expression

ibis/expr/types/generic.py

@@ -1162,11 +1162,15 @@ def __hash__(self) -> int:
     def __eq__(self, other: Value) -> ir.BooleanValue:
         if _is_null_literal(other):
             return self.isnull()
+        elif _is_null_literal(self):
+            return other.isnull()
         return _binop(ops.Equals, self, other)
 
     def __ne__(self, other: Value) -> ir.BooleanValue:
         if _is_null_literal(other):
             return self.notnull()
+        elif _is_null_literal(self):
+            return other.notnull()
         return _binop(ops.NotEquals, self, other)
 
     def __ge__(self, other: Value) -> ir.BooleanValue:

ibis/tests/expr/test_value_exprs.py

@@ -352,16 +352,14 @@ def test_notnull(table):
 
 @pytest.mark.parametrize(
     "value",
-    [
-        param(lambda: None, id="none"),
-        param(lambda: ibis.NA, id="NA"),
-        param(lambda: ibis.literal(None, type="int32"), id="typed-null"),
-    ],
+    [None, ibis.NA, ibis.literal(None, type="int32")],
+    ids=["none", "NA", "typed-null"],
 )
 def test_null_eq_and_ne(table, value):
-    other = value()
-    assert (table.a == other).equals(table.a.isnull())
-    assert (table.a != other).equals(table.a.notnull())
+    assert (table.a == value).equals(table.a.isnull())
+    assert (value == table.a).equals(table.a.isnull())
+    assert (table.a != value).equals(table.a.notnull())
+    assert (value != table.a).equals(table.a.notnull())
 
 
 @pytest.mark.parametrize("column", ["e", "f"], ids=["float32", "double"])