The constructor for JLogEntry does this:

```php
		// Sanitize category if it exists.
		if (!empty($category))
		{
			$this->category = (string) strtolower(preg_replace('/[^A-Z0-9_\.-]/i', '', $category));
		}
```

So even if you tried to use a category name with a space in it, it would be removed.