Add checks for blank cert and implement logout configs

.changeset/smooth-pigs-deny.md

@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-auth-backend': patch
+---
+
+Allow blank certificates and support logout URLs in the SAML provider.

plugins/auth-backend/config.d.ts

@@ -46,6 +46,7 @@ export interface Config {
       };
       saml?: {
         entryPoint: string;
+        logoutUrl?: string;
         issuer: string;
         cert?: string;
         privateKey?: string;

plugins/auth-backend/src/providers/saml/provider.ts

@@ -129,6 +129,7 @@ export const createSamlProvider: AuthProviderFactory = ({
   const opts = {
     callbackUrl: `${globalConfig.baseUrl}/${providerId}/handler/frame`,
     entryPoint: config.getString('entryPoint'),
+    logoutUrl: config.getOptionalString('logoutUrl'),
     issuer: config.getString('issuer'),
     cert: config.getOptionalString('cert'),
     privateCert: config.getOptionalString('privateKey'),
@@ -142,5 +143,10 @@ export const createSamlProvider: AuthProviderFactory = ({
     appUrl: globalConfig.appUrl,
   };
 
+  // passport-saml will return an error if the `cert` key is set, and the value is empty.
+  // Since we read from config (such as environment variables) an empty string should be equal to being unset.
+  if (!opts.cert) {
+    delete opts.cert;
+  }
   return new SamlAuthProvider(opts);
 };