[BUGFIX] Prevent PHP array access errors on invalid shortcut entries

If an invalid record in `sys_be_shortcuts` is stored like this:

```
INSERT INTO sys_be_shortcuts (userid,route,arguments)
     VALUES (1,'record_edit','[]');
```

then the backend cannot be accessed anymore due to PHP
errors due to unguarded array key access of the
"arguments" contents.

This patch guards the array key access to prevent such
a situation, so an invalid shortcut can be deleted from
the menu.

Database records like this might happen for migrated older
installations.

Resolves: #104496
Releases: main, 12.4, 11.5
Change-Id: I350e620eb403e53d1828cf5dedaab633ba00db55
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/85464


Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>