fix(draft): prevent NPE when draft ends during pack-passing transition

[billyhewlett]

Problem

When the final round of a draft ends while packs are still being routed
between players, BoosterDraft.nextChoice() can return null. This is
expected and documented by a comment in DraftPackPage.refresh(). However,
the null-check block was missing a return statement, causing execution to
fall through to cardManager.setPool(null), throwing a NullPointerException.

The result: a crash/error screen appears at the end of the draft. The deck
save dialog still works, but the experience is alarming to users.

Root Cause

hasNextChoice() returns true when packs are still in transit between
players (round not yet over). nextChoice() processes those pass steps
internally via a recursive passPacks() call, which can drain all remaining
packs and return null if no new round can be started. The gap between the
two checks is where the null slips through.

Fix

Add return; after parentScreen.completeDraft() in the null-pool branch
of DraftPackPage.refresh().

Testing

Reproduce by completing any cube or custom draft to the final pick. The
crash screen no longer appears.