Minor: add documentation note about NullState (#12791)

* Minor: add documentation note about `NullState`

* Remove unecessary copy/paste license

* Update datafusion/expr-common/src/groups_accumulator.rs

Author: alamb

datafusion/expr-common/src/groups_accumulator.rs

@@ -90,6 +90,11 @@ impl EmitTo {
 /// faster for queries with many group values.  See the [Aggregating Millions of
 /// Groups Fast blog] for more background.
 ///
+/// [`NullState`] can help keep the state for groups that have not seen any
+/// values and produce the correct output for those groups.
+///
+/// [`NullState`]: https://docs.rs/datafusion/latest/datafusion/physical_expr/struct.NullState.html
+///
 /// # Details
 /// Each group is assigned a `group_index` by the hash table and each
 /// accumulator manages the specific state, one per `group_index`.
@@ -117,6 +122,11 @@ pub trait GroupsAccumulator: Send {
     ///
     /// Note that subsequent calls to update_batch may have larger
     /// total_num_groups as new groups are seen.
+    ///
+    /// See [`NullState`] to help keep the state for groups that have not seen any
+    /// values and produce the correct output for those groups.
+    ///
+    /// [`NullState`]: https://docs.rs/datafusion/latest/datafusion/physical_expr/struct.NullState.html
     fn update_batch(
         &mut self,
         values: &[ArrayRef],

datafusion/functions-aggregate/src/min_max.rs

@@ -17,21 +17,6 @@
 //! [`Max`] and [`MaxAccumulator`] accumulator for the `max` function
 //! [`Min`] and [`MinAccumulator`] accumulator for the `min` function
 
-// distributed with this work for additional information
-// regarding copyright ownership.  The ASF licenses this file
-// to you under the Apache License, Version 2.0 (the
-// "License"); you may not use this file except in compliance
-// with the License.  You may obtain a copy of the License at
-//
-//   http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing,
-// software distributed under the License is distributed on an
-// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
-// KIND, either express or implied.  See the License for the
-// specific language governing permissions and limitations
-// under the License.
-
 use arrow::array::{
     ArrayRef, BinaryArray, BinaryViewArray, BooleanArray, Date32Array, Date64Array,
     Decimal128Array, Decimal256Array, Float16Array, Float32Array, Float64Array,