fix: reorder showanswer checks to restore expected behavior in preview mode

[djoseph-apphelix]

Description:

• Fixes the “Show Answer” option not working in preview mode by reordering the showanswer condition checks. This ensures preview behavior matches the expected logic without affecting published problems.

Jira:

• TNL2-446

[Copilot]

Pull request overview

This PR fixes a bug where the "Show Answer" option was not working for staff users in preview mode. The issue occurred when the showanswer field had an empty string value, which blocked staff users from seeing answers even though they should have preview access. The fix reorders the condition checks in the answer_available() method to allow staff users to bypass the empty string check while still respecting the explicit SHOWANSWER.NEVER setting.

Key changes:

• Reordered condition checks in answer_available() method to check for staff users before checking for empty string showanswer values
• Staff users can now see answers in preview mode even when showanswer == '', enabling proper preview functionality in Studio

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