A regression introduced in a recent change

With this fix a case is regressed.
ba5d235a32

If file information is not available if function is empty then line needs to be sent. With the regression empty lines were printed in error page where symbol information is not available. This fixes it. Thanks Chris for identitying this and fixing.

src/Microsoft.AspNet.Diagnostics/ErrorPageMiddleware.cs

@@ -142,7 +142,7 @@ namespace Microsoft.AspNet.Diagnostics
             int lineNumber = line.ToInt32();
 
             return string.IsNullOrEmpty(file)
-                ? LoadFrame(function, string.Empty, 0, showSource)
+                ? LoadFrame(string.IsNullOrEmpty(function) ? line.ToString() : function, string.Empty, 0, showSource)
                 : LoadFrame(function, file, lineNumber, showSource);
         }