fix(polars): make sure sum is NULL to match other backends (#11321)

Closes #11318.

Author: cpcloud

ibis/backends/polars/compiler.py

@@ -750,6 +750,14 @@ def execute_reduction(op, **kw):
     translate.register(cls, execute_reduction)
 
 
+@translate.register(ops.Sum)
+def execute_sum(op, **kw):
+    arg = translate(op.arg, **kw)
+    if (where := op.where) is not None:
+        arg = arg.filter(translate(where, **kw))
+    return pl.when(arg.count() > 0).then(arg.sum()).otherwise(None)
+
+
 @translate.register(ops.First)
 @translate.register(ops.Last)
 @translate.register(ops.Arbitrary)

ibis/backends/tests/test_aggregation.py

@@ -1731,3 +1731,12 @@ def test_group_by_scalar(alltypes, df, value):
     result = expr.execute()
     n = result["n"].values[0].item()
     assert n == len(df)
+
+
+def test_empty_sum(con):
+    t = ibis.memtable({"x": [1]}, schema={"x": "int"})
+    result = con.execute(t.count())
+    assert result == 1
+
+    result = con.to_pyarrow(t.x.sum(where=t.x > 1)).as_py()
+    assert result is None