Fix return value logic

Sources/Avatar.swift

@@ -39,14 +39,13 @@ public struct Avatar {
     }
     
     public func image(highlighted: Bool) -> UIImage {
-        guard let image = image else {
-            return placeholderImage
+        
+        switch highlighted {
+        case true:
+            return highlightedImage ?? image ?? placeholderImage
+        case false:
+            return image ?? placeholderImage
         }
         
-        guard let highlightedImage = highlightedImage else {
-            return image
-        }
-        
-        return highlighted ? image : highlightedImage
     }
 }

Sources/MessageBubble.swift

@@ -31,6 +31,6 @@ public struct MessageBubble {
     public let highlightedImage: UIImage
     
     public func image(highlighted: Bool) -> UIImage {
-        return highlighted ? image : highlightedImage
+        return highlighted ? highlightedImage : image
     }
 }