fix Blog Entity content is empty or null will throw IndexOutOfRangeException

modules/blogging/src/Volo.Blogging.Web/Pages/Blog/BloggingPage.cs

@@ -34,7 +34,10 @@ namespace Volo.Blogging.Pages.Blog
             var closingTag = "</p>";
 
             var html = RenderMarkdownToString(content);
-
+            if (string.IsNullOrWhiteSpace(html))
+            {
+                return "";
+            }
             var splittedHtml = html.Split(closingTag);
 
             if (splittedHtml.Length < 1)