Caanot get feeds content from self-hosted miniflux

### Routes

/miniflux/all

### Full routes

/miniflux/all

### Related documentation

https://docs.rsshub.app/other.html#miniflux

### What is expected?

Get Feeds content from self-hosted miniflux,
/miniflux/subscription/categories=科技, this one works;
/miniflux/all, this one not.

### What is actually happening?

Route requested: /miniflux/all
Error message: Cannot read property 'search' of undefined

### Deployment information

Self-hosted

### Deployment information (for self-hosted)

deploy to vercel, Node version: v14.18.3

### Additional info

```shell
Route requested: /miniflux/all
Error message: Cannot read property 'search' of undefined
Helpful Information to provide when opening issue:
Path: /miniflux/all
Node version: v14.18.3
```

### This is not a duplicated issue

- [X] I have searched [existing issues](https://github.com/DIYgod/RSSHub/issues) to ensure this bug has not already been reported

Start by reproducing the /miniflux/all request against a self-hosted Miniflux instance, then compare it with /miniflux/subscription/categories=科技 and read the related Miniflux documentation. Trace the route entry point that handles /miniflux/all and identify why it accesses an undefined value. Done means /miniflux/all returns feed content without the reported error.

Written by the indexing model from the issue text.