fixed policy synth (#1168)

* fixed policy synth

* fixed prettier

Author: hickeydh-aws

src/lib/cdk-accelerator/src/core/secrets-container.ts

@@ -108,6 +108,10 @@ export class SecretsContainer extends Construct {
   }
 
   protected prep(): void {
+    if (this.principals.length === 0) {
+      return;
+    }
+
     this.encryptionKey.addToResourcePolicy(
       new iam.PolicyStatement({
         actions: ['kms:Decrypt'],