"Create File System Warm Storage Repository" with space results in a 500

## Expected Behavior
Creating a File System Warm Storage Repository for an index set should either accept names with spaces or provide a clear validation error preventing invalid input before submission.

## Current Behavior
When creating a File System Warm Storage Repository and using a name that contains spaces, the system returns a 500 Internal Server Error instead of handling the input gracefully.
```
{"type":"ApiError","message":"Cannot throw exception because \"e\" is null"}
```

## Possible Solution
Add frontend and/or backend validation to reject invalid repository names (e.g., containing spaces) with a descriptive error message. Alternatively, sanitize or support such names if technically feasible.

## Steps to Reproduce (for bugs)
1. Go through the Input Setup Wizard
2. Create a new Index Set
3. Select "Create File System Warm Storage Repository"
4. Enter a repository name that contains spaces
5. Submit the form
6. Observe a 500 Internal Server Error

## Context
This occurs during initial setup flows and can block users from completing index configuration. The lack of validation makes the failure unexpected and harder to diagnose.

## Your Environment
* Graylog Version: Graylog 7.1.0-beta 1

## Checklist
[] This issue fix need to be backported.
[] Does this issue have **security** implications?