Fix loading custom rules that do not have comment help

Add conditions to check if indexing does not take place on a null valued Value property.

If a PowerShell based custom rule does't have comment based help, the engine throws an error. This occurs because the lack of comment based help results in a null value for the Value property of the dynamically typed description variable.

Author: Kapil Borle

Engine/ScriptAnalyzer.cs

@@ -764,8 +764,8 @@ private List<ExternalRule> GetExternalRule(string[] moduleNames)
                             {
                                 dynamic description = helpContent[0].Properties["Description"];
 
-                                if (null != description)
-                                {
+                                if (null != description && null != description.Value && description.Value.GetType().IsArray)
+                                {                                    
                                     desc = description.Value[0].Text;
                                 }
                             }