airbrake: fix config to be optional, unbreak local dev startup

Signed-off-by: Fredrik Adelöw <freben@gmail.com>

.changeset/tidy-otters-roll.md

@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-airbrake-backend': patch
+---
+
+Make config optional

plugins/airbrake-backend/config.d.ts

@@ -16,7 +16,7 @@
 
 export interface Config {
   /** Configuration options for the Airbrake plugin */
-  airbrake: {
+  airbrake?: {
     /**
      * The API Key to authenticate requests. More details on how to get this here: https://airbrake.io/docs/api/#authentication
      * @visibility secret