Handle search keywords containing characters outside the latin alphabet

• This is not a feature request
• This is not an image-provider request
• I have searched the issuetracker if the issue is not already reported.

Describe the bug:
Images containing umlaut characters (e.g. Ä or Ö) in their tags aren't displaying.

To Reproduce:
Steps to reproduce the behavior:

1. Go to Splash settings
2. List only tags containing umlaut characters, for example jönköping, hämeenlinna, jyväskylä
3. Refresh the page, and an error background image is displayed.

Expected behavior:
Images containing these tags should appear.

Server:

• Nextcloud version: 27.0.1
• Splash-App version: 2.2.1

Additional Information:

• The bug can be circumvented by omitting the umlauts, i.e. "jonkoping, hameenlinna, jyvaskyla".
• It looks like some tags containing umlauts DO display other images besides the error image, such as öland and häme. Note, however, that the tag häme only displays an image of a cat, completely different from the 23 images that you can find, if you use the same tag at unsplash.com.
• I also tried adding cyrillic tags, such as Москва, and it seemed to work.

No file, test, or entry point is named. Start by reproducing the issue from Splash settings with tags such as jönköping or Москва, then trace the search request and response handling. Done means tags containing non-Latin characters return the expected images without an error background.