Fix long-living effect leak (#1338)

`Store.send` is currently spinning up a task for every batch of actions
sent through it, even if no effects are in-flight. This issue compounds
itself recursively when long-living effects feed actions back into the
store.

Instead, let's only spin up tasks when we need to.

Fixes #1337

(cherry picked from commit e407d7e418cff6b75f4d052d53bc679781b19885)

Author: stephencelis

Sources/ComposableArchitecture/Store.swift

@@ -408,6 +408,7 @@ public final class Store<State, Action> {
       }
     }
 
+    guard !tasks.wrappedValue.isEmpty else { return nil }
     return Task {
       await withTaskCancellationHandler {
         var index = tasks.wrappedValue.startIndex