Update stylecop.json schema to have correct default for headerDecoration.

The default is documented to be the empty string, and since it's typed as `string`, `null` is not actually a valid value and will fail to validate.

Author: alexrp

StyleCop.Analyzers/StyleCop.Analyzers/Settings/stylecop.schema.json

@@ -263,7 +263,7 @@
             "headerDecoration": {
               "type": "string",
               "description": "The text used as decoration for the copyright header comment.",
-              "default": null
+              "default": ""
             },
             "fileNamingConvention": {
               "type": "string",