Convert an if used for early return into a guard

`guard` is better for early return because the compiler enforces calling
`return`.

Author: mokagio

WordPress/Classes/ViewRelated/Me/App Settings/App Icons/AppIconListViewModel.swift

@@ -39,10 +39,12 @@ final class AppIconListViewModel: AppIconListViewModelType {
             .sorted(by: sortWithPriority(toItemsWithPrefix: AppIcon.defaultIconName))
         let legacyIcons = {
             let icons = allIcons.filter({ $0.isLegacy == true })
-            if let legacyIconName = AppIcon.defaultLegacyIconName {
-                return icons.sorted(by: sortWithPriority(toItemsWithPrefix: legacyIconName))
+
+            guard let legacyIconName = AppIcon.defaultLegacyIconName else {
+                return icons
             }
-            return icons
+
+            return icons.sorted(by: sortWithPriority(toItemsWithPrefix: legacyIconName))
         }()
 
         // Set icons