Gravatar download: handle successful response for unknown user

WordPressKit.podspec

@@ -1,6 +1,6 @@
 Pod::Spec.new do |s|
   s.name          = "WordPressKit"
-  s.version       = "4.7.1-beta.1"
+  s.version       = "4.8.0-beta.1"
   s.summary       = "WordPressKit offers a clean and simple WordPress.com and WordPress.org API."
 
   s.description   = <<-DESC

WordPressKit/GravatarServiceRemote.swift

@@ -35,15 +35,24 @@ open class GravatarServiceRemote {
             }
             do {
                 let jsonData = try JSONSerialization.jsonObject(with: data, options: .allowFragments)
-                if let jsonDictionary = jsonData as? [String: Array<Any>],
+
+                guard let jsonDictionary = jsonData as? [String: Array<Any>],
                     let entry = jsonDictionary["entry"],
-                    let profileData = entry.first as? NSDictionary {
-                    let profile = RemoteGravatarProfile(dictionary: profileData)
-                    DispatchQueue.main.async {
-                        success(profile)
-                    }
-                    return
+                    let profileData = entry.first as? NSDictionary else {
+                        DispatchQueue.main.async {
+                            // This case typically happens when the endpoint does
+                            // successfully return but doesn't find the user.
+                            failure(nil)
+                        }
+                        return
                 }
+
+                let profile = RemoteGravatarProfile(dictionary: profileData)
+                DispatchQueue.main.async {
+                    success(profile)
+                }
+                return
+
             } catch {
                 failure (error)
                 return