chore: remove cast string

Author: Vu Van Duc

src/keri/app/clienting.ts

@@ -179,7 +179,7 @@ export class SignifyClient {
         let signed_headers = new Headers();
         const final_headers = new Headers();
 
-        headers.set('Signify-Resource', String(this.controller.pre));
+        headers.set('Signify-Resource', this.controller.pre!);
         headers.set(
             HEADER_SIG_TIME,
             new Date().toISOString().replace('Z', '000+00:00')

src/keri/app/controller.ts

@@ -232,8 +232,8 @@ export class Controller {
         const sn =
             new CesrNumber({}, undefined, String(this.serder.sad['s'])).num + 1;
         this.serder = interact({
-            pre: this.serder.pre || '',
-            dig: this.serder.sad['d'] || '',
+            pre: this.serder.pre!,
+            dig: this.serder.sad['d']!,
             sn: sn,
             data: [anchor],
             version: Vrsn_1_0,