Dendrite responds incorrectly to invalid media

*This issue was originally created by [**@ShadowJonathan**](https://github.com/ShadowJonathan) at .*

### Background information

- **Dendrite version or git SHA**: `221ed52`
- **Monolith or Polylith?**: Monolith
- **SQLite3 or Postgres?**: Postgres
- **Running in Docker?**: No
- **`go version`**: Go 1.17

Homeserver in question is `jp.cube64128.xyz`

These same symptoms are present on `dendrite.matrix.org`

### Description

Dendrite returns a confusing error message to non-existent media;

`{"errcode":"M_NOT_FOUND","error":"Failed to download: fileutils.GetPathFromBase64Hash: Invalid filePath (Base64Hash too short - min 3 characters): \"\""}`

[this MXC](https://jp.cube64128.xyz/_matrix/media/r0/download/matrix.org/DRevoaEiuzbkOznknySKuMmE) is valid, while [this one](https://jp.cube64128.xyz/_matrix/media/r0/download/matrix.org/DRevoaEiuzbkOznknySKuMmF) isn't, replacing the latter with `matrix.org` or any synapse server returns a proper message of "not found"

Reproduce the two Matrix media download URLs in the issue and compare their responses with Dendrite and Synapse. Trace the media download handling from the reported endpoint; done means an invalid MXC returns a clear not-found response rather than the internal Base64Hash error.

Written by the indexing model from the issue text.