techdocs: make builder optional

Signed-off-by: Vincenzo Scamporlino <vincenzos@spotify.com>

plugins/techdocs-backend/config.d.ts

@@ -24,7 +24,7 @@ export interface Config {
      * Documentation building process depends on the builder attr
      * @visibility frontend
      */
-    builder: 'local' | 'external';
+    builder?: 'local' | 'external';
 
     /**
      * Techdocs generator information

plugins/techdocs-backend/src/service/DefaultDocsBuildStrategy.ts

@@ -29,6 +29,8 @@ export class DefaultDocsBuildStrategy implements DocsBuildStrategy {
   }
 
   async shouldBuild(_: { entity: Entity }): Promise<boolean> {
-    return this.config.getString('techdocs.builder') === 'local';
+    return [undefined, 'local'].includes(
+      this.config.getOptionalString('techdocs.builder'),
+    );
   }
 }