pixiv: fix nil commentary fields to empty string.

app/logical/pixiv_api_client.rb

@@ -94,8 +94,8 @@ class PixivApiClient
       @user_id = json["user"]["id"]
       @moniker = json["user"]["account"]
       @page_count = json["page_count"].to_i
-      @artist_commentary_title = json["title"]
-      @artist_commentary_desc = json["caption"]
+      @artist_commentary_title = json["title"].to_s
+      @artist_commentary_desc = json["caption"].to_s
       @tags = [json["tags"], json["tools"]].flatten.compact.reject {|x| x =~ /^http:/}
 
       if page_count > 1
@@ -173,4 +173,4 @@ private
       access_token
     end
   end
-end
+end