Return undefined rather than an empty string

Signed-off-by: Iain Billett <iain@roadie.io>

.changeset/chatty-pens-try.md

@@ -2,5 +2,5 @@
 '@backstage/integration': patch
 ---
 
-Return an empty token rather than fail where the owner is not in the allowed installation owners
+Do not return a token rather than fail where the owner is not in the allowed installation owners
 for a GitHub app. This allows anonymous access to public files in the organisation.

packages/integration/src/github/SingleInstanceGithubCredentialsProvider.ts

@@ -92,11 +92,11 @@ class GithubAppManager {
   async getInstallationCredentials(
     owner: string,
     repo?: string,
-  ): Promise<{ accessToken: string }> {
+  ): Promise<{ accessToken: string | undefined }> {
     const { installationId, suspended } = await this.getInstallationData(owner);
     if (this.allowedInstallationOwners) {
       if (!this.allowedInstallationOwners?.includes(owner)) {
-        return { accessToken: '' }; // An empty token allows anonymous access to public repos
+        return { accessToken: undefined }; // An empty token allows anonymous access to public repos
       }
     }
     if (suspended) {