[lua] Fix CoP 1-3 Promy Entry

[Aeshur]

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?

Fixes a bug that would cause players zoning into their 3rd Promy/Hall of Transference entry to have a black screen. Also adds a guard against prevZone nil indexing (e.g. exiting from inside the Promy back to Hall).

Steps to test these changes

Do CoP 1-3 and zone into 3rd set of Hall of Transference after completing the first 2.

[WinterSolstice8]

Good catch, thanks

[Xaver-DaRed]