fix(medusa): Error handling middleware should be applied when no config is exported (#5634)

**What**
- Fixes an issue where the default errorHandler middleware was not applied if the user did not have a `middlewares.ts` file.
- Updates the type of `MiddlewaresConfig` so `middlewares` is now optional, as people might want to only set a bodyParser config for a matcher and not provide any additional middleware.

.changeset/smooth-baboons-repeat.md

@@ -0,0 +1,5 @@
+---
+"@medusajs/medusa": patch
+---
+
+fix(medusa): Ensure that errorHandler middleware is applied if no middleware config exists

packages/medusa/src/loaders/helpers/routing/index.ts

@@ -653,12 +653,7 @@ export class RoutesLoader {
    */
   applyErrorHandlerMiddleware(): void {
     const middlewareDescriptor = this.globalMiddlewaresDescriptor
-
-    if (!middlewareDescriptor) {
-      return
-    }
-
-    const errorHandlerFn = middlewareDescriptor.config?.errorHandler
+    const errorHandlerFn = middlewareDescriptor?.config?.errorHandler
 
     /**
      * If the user has opted out of the error handler then return
@@ -740,6 +735,10 @@ export class RoutesLoader {
      */
 
     for (const route of routes) {
+      if (!route.middlewares || !route.middlewares.length) {
+        continue
+      }
+
       const methods = (
         Array.isArray(route.method) ? route.method : [route.method]
       ).filter(Boolean) as MiddlewareVerb[]

packages/medusa/src/loaders/helpers/routing/types.ts

@@ -65,7 +65,7 @@ export type MiddlewareRoute = {
   method?: MiddlewareVerb | MiddlewareVerb[]
   matcher: string | RegExp
   bodyParser?: ParserConfig
-  middlewares: MiddlewareFunction[]
+  middlewares?: MiddlewareFunction[]
 }
 
 export type MiddlewaresConfig = {