Merge pull request #2043 from dreampiggy/set_uibutton_placeholder_if_url_is_nil

Set UIButton's placeholer-image even if the url is nil

@@ -71,10 +71,9 @@ static inline NSString * backgroundImageURLKeyForState(UIControlState state) {
                  completed:(nullable SDExternalCompletionBlock)completedBlock {
     if (!url) {
         [self.imageURLStorage removeObjectForKey:imageURLKeyForState(state)];
-        return;
-    }
-    
+    } else {
         self.imageURLStorage[imageURLKeyForState(state)] = url;
+    }
     
     __weak typeof(self)weakSelf = self;
     [self sd_internalSetImageWithURL:url
@@ -131,10 +130,9 @@ static inline NSString * backgroundImageURLKeyForState(UIControlState state) {
                            completed:(nullable SDExternalCompletionBlock)completedBlock {
     if (!url) {
         [self.imageURLStorage removeObjectForKey:backgroundImageURLKeyForState(state)];
-        return;
-    }
-    
+    } else {
         self.imageURLStorage[backgroundImageURLKeyForState(state)] = url;
+    }
     
     __weak typeof(self)weakSelf = self;
     [self sd_internalSetImageWithURL:url