Change namespace contains_nulls to struct (#8523)

This PR just changes the `contains_nulls` namespace to be `struct` instead in order to remove it from the namespace list in the libcudf doxygen documentation.

![namespace-list](https://user-images.githubusercontent.com/45795991/122110272-58292700-cdec-11eb-9eb5-818b0f3ef1b8.png)

Changing this to `struct` does not require any other code changes.

Authors:
  - David Wendt (https://github.com/davidwendt)

Approvers:
  - Robert Maynard (https://github.com/robertmaynard)
  - Paul Taylor (https://github.com/trxcllnt)

URL: #8523

cpp/include/cudf/column/column_device_view.cuh

@@ -54,14 +54,14 @@ namespace cudf {
  * - `DYNAMIC` defers the assumption of nullability to runtime with the users stating
  *    on construction of the iterator if column has nulls.
  */
-namespace contains_nulls {
-struct YES {
-};
-struct NO {
-};
-struct DYNAMIC {
+struct contains_nulls {
+  struct YES {
+  };
+  struct NO {
+  };
+  struct DYNAMIC {
+  };
 };
-}  // namespace contains_nulls
 
 namespace detail {
 /**