don't allocate empty streams in Task::batch

Author: edwloef

runtime/src/task.rs

@@ -108,7 +108,11 @@ impl<T> Task<T> {
         }
 
         Self {
-            stream: Some(boxed_stream(select_all)),
+            stream: if select_all.is_empty() {
+                None
+            } else {
+                Some(boxed_stream(select_all))
+            },
             units,
         }
     }