optional chaining to prevent error during ng add

If there is no tslint.json an error occurs. With optional chaining we can catch that. 

Closes Issue #2

Author: mcanoglu

libs/ddd/src/schematics/utils/update-linting-rules.ts

@@ -32,7 +32,7 @@ export function checkRuleExists(rules: object, context: SchematicContext) {
 
 export function addDomainToLintingRules(domainName: string): Rule {
   return (host: Tree, context: SchematicContext) => {
-    const text = host.read('tslint.json').toString();
+    const text = host.read('tslint.json')?.toString();
     const rules = JSON.parse(text);
 
     if (!checkRuleExists(rules, context)) return;
@@ -97,4 +97,4 @@ export function initLintingRules(): Rule {
     const newText = JSON.stringify(rules, undefined, 2);
     host.overwrite('tslint.json', newText);
   }
-}
+}