fix(config): check for undefined str when validating handlebars helpers (#16047)

lib/util/template/index.ts

@@ -17,10 +17,10 @@ handlebars.registerHelper(
     (context || '').replace(new RegExp(find, 'g'), replace) // TODO #12873
 );
 
-handlebars.registerHelper('lowercase', (str: string) => str.toLowerCase());
+handlebars.registerHelper('lowercase', (str: string) => str?.toLowerCase());
 
-handlebars.registerHelper('containsString', (str, subStr, options) =>
-  str.includes(subStr)
+handlebars.registerHelper('containsString', (str, subStr) =>
+  str?.includes(subStr)
 );
 
 handlebars.registerHelper({