Fix the issue for WebImage when url is nil will not cause the reloading

Example/SDWebImageSwiftUIDemo/ContentView.swift

@@ -17,6 +17,43 @@ class UserSettings: ObservableObject {
     #endif
 }
 
+// Test Switching nil url
+struct ContentView: View {
+    @State var isOn = false
+    @State var animated: Bool = false // You can change between WebImage/AnimatedImage
+
+    var url: URL? {
+        if isOn {
+            .init(string: "https://upload.wikimedia.org/wikipedia/commons/thumb/c/c1/Google_%22G%22_logo.svg/1024px-Google_%22G%22_logo.svg.png")
+        } else {
+            nil
+        }
+    }
+
+    var body: some View {
+        VStack {
+            Text("\(animated ? "AnimatedImage" : "WebImage")")
+            Spacer()
+            if animated {
+                AnimatedImage(url: url)
+                    .resizable()
+                    .scaledToFit()
+                    .frame(width: 100, height: 100)
+            } else {
+                WebImage(url: url)
+                    .resizable()
+                    .scaledToFit()
+                    .frame(width: 100, height: 100)
+            }
+            Button("Toggle \(isOn ? "nil" : "valid") URL") {
+                isOn.toggle()
+            }
+            Spacer()
+            Toggle("Switch", isOn: $animated)
+        }
+    }
+}
+
 // Test Switching url using @State
 struct ContentView2: View {
     @State var imageURLs = [
@@ -63,7 +100,7 @@ struct ContentView2: View {
     }
 }
 
-struct ContentView: View {
+struct ContentView3: View {
     @State var imageURLs = [
     "http://assets.sbnation.com/assets/2512203/dogflops.gif",
     "https://raw.githubusercontent.com/liyong03/YLGIFImage/master/YLGIFImageDemo/YLGIFImageDemo/joy.gif",

SDWebImageSwiftUI/Classes/WebImage.swift

@@ -163,6 +163,7 @@ public struct WebImage<Content> : View where Content: View {
                 }
             } else {
                 content((imageManager.error != nil) ? .failure(imageManager.error!) : .empty)
+                setupPlaceholder()
                 // Load Logic
                 .onPlatformAppear(appear: {
                     self.setupManager()
@@ -326,6 +327,14 @@ public struct WebImage<Content> : View where Content: View {
             }
         }
     }
+    
+    /// Placeholder View Support
+    func setupPlaceholder() -> some View {
+        let result = content((imageManager.error != nil) ? .failure(imageManager.error!) : .empty)
+        // Custom ID to avoid SwiftUI engine cache the status, and does not call `onAppear` when placeholder not changed (See `ContentView.swift/ContentView2` case)
+        // Because we load the image url in placeholder's `onAppear`, it should be called to sync with state changes :)
+        return result.id(imageModel.url)
+    }
 }
 
 // Layout