asyncChannel: fixes potential crashes when no more awaiting

This commit fixes crashes where the state is "awaiting" with no more awaiting clients.
The state is now set to .idle to reflect the reality.

Author: Thibault Wittemberg

Sources/AsyncAlgorithms/AsyncChannel.swift

@@ -89,7 +89,11 @@ public final class AsyncChannel<Element: Sendable>: AsyncSequence, Sendable {
       switch self {
       case .awaiting(var awaiting):
         let continuation = awaiting.remove(Awaiting(placeholder: generation))?.continuation
-        self = .awaiting(awaiting)
+        if awaiting.isEmpty {
+          self = .idle
+        } else {
+          self = .awaiting(awaiting)
+        }
         return continuation
       case .idle:
         self = .awaiting([Awaiting(cancelled: generation)])
@@ -145,7 +149,11 @@ public final class AsyncChannel<Element: Sendable>: AsyncSequence, Sendable {
             nexts.remove(Awaiting(placeholder: generation))
             cancelled = true
           }
-          state.emission = .awaiting(nexts)
+          if nexts.isEmpty {
+            state.emission = .idle
+          } else {
+            state.emission = .awaiting(nexts)
+          }
           return nil
         }
       }?.resume()