fix clippy

Author: xudong963

datafusion/physical-plan/src/aggregates/group_values/mod.rs

@@ -119,7 +119,7 @@ pub trait GroupValues: Send {
 ///   - If group by single column, and type of this column has
 ///     the specific [`GroupValues`] implementation, such implementation
 ///     will be chosen.
-///   
+///
 ///   - If group by multiple columns, and all column types have the specific
 ///     `GroupColumn` implementations, `GroupValuesColumn` will be chosen.
 ///

datafusion/physical-plan/src/aggregates/group_values/multi_group_by/mod.rs

@@ -91,6 +91,11 @@ pub trait GroupColumn: Send + Sync {
     /// Returns the number of rows stored in this builder
     fn len(&self) -> usize;
 
+    /// true if len == 0
+    fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
     /// Returns the number of bytes used by this [`GroupColumn`]
     fn size(&self) -> usize;