[LUA] Fix Despoil incorrect message on fail

[ampitere]

I affirm:

• I understand that if I do not agree to the following points by completing the checkboxes my PR will be ignored.
• I understand I should leave resolving conversations to the LandSandBoat team so that reviewers won't miss what was said.
• I have read and understood the Contributing Guide and the Code of Conduct.
• I have tested my code and the things my code has changed since the last commit in the PR and will test after any later commits.

What does this pull request do?

Despoil currently gives an improper message when it fails due to a stolen always having a value. When a mob has no item to steal or it has already been stolen the stolen variable will be 0.

Before

After

Steps to test these changes

1. !changejob 6 99
2. Find a mob
3. Attempt to Despoil from it