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

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