Support init with optional url

Author: cozzin

Example/SDWebImageSwiftUIDemo/ContentView.swift

@@ -14,10 +14,10 @@ struct ContentView: View {
 
     var body: some View {
         VStack {
-            WebImage(url: URL(string: "https://nokiatech.github.io/heif/content/images/ski_jump_1440x960.heic")!)
+            WebImage(url: URL(string: "https://nokiatech.github.io/heif/content/images/ski_jump_1440x960.heic"))
                 .scaledToFit()
                 .frame(width: 300, height: 300, alignment: .center)
-            AnimatedImage(url: URL(string: "https://raw.githubusercontent.com/liyong03/YLGIFImage/master/YLGIFImageDemo/YLGIFImageDemo/joy.gif")!)
+            AnimatedImage(url: URL(string: "https://raw.githubusercontent.com/liyong03/YLGIFImage/master/YLGIFImageDemo/YLGIFImageDemo/joy.gif"))
 //                .scaledToFit() // Apple's Bug ? Custom UIView does not passthrough the `contentMode` from Swift UI layout system into UIKit layout system
                 .frame(width: 400, height: 300, alignment: .center)
         }

SDWebImageSwiftUI/Classes/AnimatedImage.swift

@@ -65,7 +65,7 @@ public struct AnimatedImage: ViewRepresentable {
         }
     }
 
-    public init(url: URL, placeholder: Image? = nil, options: SDWebImageOptions = [], context: [SDWebImageContextOption : Any]? = nil) {
+    public init(url: URL?, placeholder: Image? = nil, options: SDWebImageOptions = [], context: [SDWebImageContextOption : Any]? = nil) {
         self.url = url
     }
 

SDWebImageSwiftUI/Classes/ImageManager.swift

@@ -26,11 +26,11 @@ class ImageManager : ObservableObject {
         }
     }
 
-    var url: URL
+    var url: URL?
     var options: SDWebImageOptions
     var context: [SDWebImageContextOption : Any]?
 
-    init(url: URL, options: SDWebImageOptions = [], context: [SDWebImageContextOption : Any]? = nil) {
+    init(url: URL?, options: SDWebImageOptions = [], context: [SDWebImageContextOption : Any]? = nil) {
         self.url = url
         self.options = options
         self.context = context

SDWebImageSwiftUI/Classes/WebImage.swift

@@ -10,14 +10,14 @@ import SwiftUI
 import SDWebImage
 
 public struct WebImage : View {
-    public var url: URL
+    public var url: URL?
     public var placeholder: Image?
     public var options: SDWebImageOptions
     public var context: [SDWebImageContextOption : Any]?
 
     @ObservedObject var imageManager: ImageManager
 
-    public init(url: URL, placeholder: Image? = nil, options: SDWebImageOptions = [], context: [SDWebImageContextOption : Any]? = nil) {
+    public init(url: URL?, placeholder: Image? = nil, options: SDWebImageOptions = [], context: [SDWebImageContextOption : Any]? = nil) {
         self.url = url
         self.placeholder = placeholder
         self.options = options