Revert "Return "invalid" stores from scoping (#2601)"

This reverts commit ff27687.

This introduced a regression in our examples, so we should revisit in a
new PR with more test coverage.

Sources/ComposableArchitecture/Store.swift

@@ -139,7 +139,7 @@ public final class Store<State, Action> {
   private var isSending = false
   var parentCancellable: AnyCancellable?
   private let reducer: any Reducer<State, Action>
-  @_spi(Internals) public var stateSubject: CurrentValueSubject<State, Never>!
+  @_spi(Internals) public var stateSubject: CurrentValueSubject<State, Never>
   #if DEBUG
     private let mainThreadChecksEnabled: Bool
   #endif
@@ -175,14 +175,6 @@ public final class Store<State, Action> {
     }
   }
 
-  fileprivate init() {
-    self._isInvalidated = { true }
-    self.reducer = EmptyReducer()
-    #if DEBUG
-      self.mainThreadChecksEnabled = true
-    #endif
-  }
-
   deinit {
     self.invalidate()
     Logger.shared.log("\(storeTypeName(of: self)).deinit")
@@ -1002,15 +994,12 @@ private final class ScopedStoreReducer<RootState, RootAction, State, Action>: Re
 
   @inlinable
   func reduce(into state: inout State, action: Action) -> Effect<Action> {
-    let isInvalid = self.isInvalid()
-    if isInvalid {
+    if self.isInvalid() {
       self.onInvalidate()
     }
     self.isSending = true
     defer {
-      if !isInvalid || state is _OptionalProtocol {
-        state = self.toState(self.rootStore.stateSubject.value)
-      }
+      state = self.toState(self.rootStore.stateSubject.value)
       self.isSending = false
     }
     if let action = self.fromAction(action),
@@ -1043,10 +1032,6 @@ extension ScopedStoreReducer: AnyScopedStoreReducer {
     isInvalid: ((S) -> Bool)?,
     removeDuplicates isDuplicate: ((ChildState, ChildState) -> Bool)?
   ) -> Store<ChildState, ChildAction> {
-    guard isInvalid.map({ $0(store.stateSubject.value) == false }) ?? true
-    else {
-      return Store()
-    }
     let id = id?(store.stateSubject.value)
     if let id = id,
       let childStore = store.children[id] as? Store<ChildState, ChildAction>
@@ -1068,7 +1053,7 @@ extension ScopedStoreReducer: AnyScopedStoreReducer {
     }
     let reducer = ScopedStoreReducer<RootState, RootAction, ChildState, ChildAction>(
       rootStore: self.rootStore,
-      state: { [stateSubject = store.stateSubject!] _ in toChildState(stateSubject.value) },
+      state: { [stateSubject = store.stateSubject] _ in toChildState(stateSubject.value) },
       action: { fromChildAction($0).flatMap(fromAction) },
       isInvalid: isInvalid,
       onInvalidate: { [weak store] in