Add null to GetJsonCompatibleString

Author: darrelmiller

src/Microsoft.OpenApi/Writers/SpecialCharacterStringExtensions.cs

@@ -196,6 +196,11 @@ internal static string GetYamlCompatibleString(this string input)
         /// </summary>
         internal static string GetJsonCompatibleString(this string value)
         {
+            if (value == null)
+            {
+                return "null";
+            }
+
             // Show the control characters as strings
             // http://json.org/