[Impeller] vulkan: the back arrow isn't rendering in the gallery

[gaaclarke]

engine version

commit 622751bdce53a8bc95ee933a13eccbabd23df913 (HEAD, upstream/master, upstream/main)
Author: Zachary Anderson <zanderso@users.noreply.github.com>
Date:   Mon Jun 5 08:38:32 2023 -0700

    Check more optional accesses (#42528)
    
    Related https://github.com/flutter/flutter/issues/127701

screenshot

[jonahwilliams]

The back arrow is from an Icon font, so this is a dupe of #128283

[github-actions]

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