feat(api): count nulls with topk

BREAKING CHANGE: `expr.topk(...)` now includes null counts. The row count of the `topk` call will not differ, but the number of nulls counted will no longer be zero. To drop the `null` row use the `dropna` method.

ibis/expr/types/generic.py

@@ -1827,10 +1827,11 @@ def topk(
             raise com.IbisTypeError("TopK must depend on exactly one table.")
 
         table = table.to_expr()
+
         if by is None:
-            metric = self.count()
-        else:
-            (metric,) = bind(table, by)
+            by = lambda t: t.count()
+
+        (metric,) = bind(table, by)
 
         return table.aggregate(metric, by=[self]).order_by(metric.desc()).limit(k)