Fix screen interstitial null key

[nolanpro]

Issue & Reproduction Steps

Importing a process with in update mode was erasing the default screen interstitial key

Solution

• When importing, do not set to null unless there is already another screen with the interstitial key
• Create an upgrade script to fix instances where the key was erased

How to Test

• Run php artisan upgrade
• In the DB, ensure you have one screen with the interstitial key
• Create a process with a task using the default screen interstitial
• Export the process
• Import the process, using "Basic" mode and the "update" button
• Ensure the same screen still has the interstitial key

Related Tickets & Packages

• https://processmaker.atlassian.net/browse/FOUR-20043

Code Review Checklist

• I have pulled this code locally and tested it on my instance, along with any associated packages.
• This code adheres to ProcessMaker Coding Guidelines.
• This code includes a unit test or an E2E test that tests its functionality, or is covered by an existing test.
• This solution fixes the bug reported in the original ticket.
• This solution does not alter the expected output of a component in a way that would break existing Processes.
• This solution does not implement any breaking changes that would invalidate documentation or cause existing Processes to fail.
• This solution has been tested with enterprise packages that rely on its functionality and does not introduce bugs in those packages.
• This code does not duplicate functionality that already exists in the framework or in ProcessMaker.
• This ticket conforms to the PRD associated with this part of ProcessMaker.