bug: passing a wrong theme now causes 500 Internal Error

### Describe the bug

When passing a theme that does not exist in query params (`theme=...`), I get the following error:

```
500: INTERNAL_SERVER_ERROR
Code: FUNCTION_INVOCATION_FAILED
ID: fra1::hrdcg-1762421377288-2993a3767f96
```

### Expected behavior

I'd like to have the default theme as a fallback.

### Screenshots / Live demo link

[https://github-readme-stats.vercel.app/api?username=mikyll&show_icons=not&**theme=foobar**](https://github-readme-stats.vercel.app/api?username=mikyll&show_icons=not&theme=foobar)

### Additional context

I noticed this some day ago, so I think this bug was mistakenly introduced with most recent commits

Start by reproducing the linked API request with an invalid theme query parameter, then trace where the theme is selected and where the default is applied. Done means an unknown theme returns a response using the default theme instead of a 500 Internal Error; add or update coverage if the relevant tests are found.

Written by the indexing model from the issue text.