techdocs: make builder optional

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

plugins/techdocs/src/client.ts

@@ -151,7 +151,7 @@ export class TechDocsStorageClient implements TechDocsStorageApi {
   }
 
   async getBuilder(): Promise<string> {
-    return this.configApi.getString('techdocs.builder');
+    return this.configApi.getOptionalString('techdocs.builder') || 'local';
   }
 
   /**

plugins/techdocs/src/reader/components/TechDocsNotFound.tsx

@@ -39,7 +39,7 @@ export const TechDocsNotFound = ({ errorMessage }: Props) => {
   }, [analyticsApi, entityRef, location]);
 
   let additionalInfo = '';
-  if (techdocsBuilder !== 'local') {
+  if (![undefined, 'local'].includes(techdocsBuilder)) {
     additionalInfo =
       "Note that techdocs.builder is not set to 'local' in your config, which means this Backstage app will not " +
       "generate docs if they are not found. Make sure the project's docs are generated and published by some external " +