Change to use the empty Image as static variable

Author: dreampiggy

SDWebImageSwiftUI/Classes/SDWebImageSwiftUI.swift

@@ -26,6 +26,8 @@ extension Image {
         self.init(uiImage: platformImage)
         #endif
     }
+    
+    static var empty = Image(platformImage: PlatformImage())
 }
 
 #if os(macOS)

SDWebImageSwiftUI/Classes/WebImage.swift

@@ -12,7 +12,6 @@ import SDWebImage
 /// A Image View type to load image from url. Supports static/animated image format.
 @available(iOS 13.0, OSX 10.15, tvOS 13.0, watchOS 6.0, *)
 public struct WebImage : View {
-    static var emptyImage = PlatformImage()
     var configurations: [(Image) -> Image] = []
 
     var placeholder: AnyView?
@@ -113,7 +112,7 @@ public struct WebImage : View {
                     } else {
                         // Should not use `EmptyView`, which does not respect to the container's frame modifier
                         // Using a empty image instead for better compatible
-                        configurations.reduce(Image(platformImage: WebImage.emptyImage)) { (previous, configuration) in
+                        configurations.reduce(Image.empty) { (previous, configuration) in
                             configuration(previous)
                         }
                     }

Tests/AnimatedImageTests.swift

@@ -150,7 +150,7 @@ class AnimatedImageTests: XCTestCase {
             XCTAssert(view.isKind(of: SDAnimatedImageView.self))
             XCTAssertEqual(context.coordinator.userInfo?["foo"] as? String, "bar")
         }
-        .placeholder(WebImage.emptyImage)
+        .placeholder(PlatformImage())
         .indicator(SDWebImageActivityIndicator.medium)
         // Image
         .resizable()