Use attribute name as value if value is null

src/Markdig/Renderers/HtmlRenderer.cs

@@ -314,12 +314,9 @@ namespace Markdig.Renderers
                 foreach (var property in attributes.Properties)
                 {
                     Write(" ").Write(property.Key);
-                    if (property.Value != null)
-                    {
-                        Write("=").Write("\"");
-                        WriteEscape(property.Value);
-                        Write("\"");
-                    }
+                    Write("=").Write("\"");
+                    WriteEscape(property.Value ?? property.Key);
+                    Write("\"");
                 }
             }
 
@@ -364,4 +361,4 @@ namespace Markdig.Renderers
             return this;
         }
    }
-}
+}