Add NullState::is_null

datafusion/physical-expr-common/src/aggregate/groups_accumulator/accumulate.rs

@@ -324,6 +324,12 @@ impl NullState {
         }
     }
 
+    /// Check if the accumulated value for the group at the given `index` is `null`,
+    /// meaning that no values passing the filter were seen yet for this group.
+    pub fn is_null(&self, index: usize) -> bool {
+        !self.seen_values.get_bit(index)
+    }
+
     /// Creates the a [`NullBuffer`] representing which group_indices
     /// should have null values (because they never saw any values)
     /// for the `emit_to` rows.