BUG: Custom questions "machine-name" allows empty input

## 🐛 Bug Report

### What went wrong? 🤔

The "machine-name" field in custom questions is a required field but it lets you pass empty to it. This breaks registrations and event view, and probably some others as well

### Expected Behavior & Solution

Frontend and backend should check if the "machine-name" is not null, and throw an error if the field is left empty.

Locate the frontend and backend handling for custom questions, focusing on validation of the "machine-name" field. Reproduce an empty value, then verify that both sides reject it and that registrations and event views no longer break.

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