[ChromeVox] Fix 'Custom' Braille Display in new ChromeVox Settings Page

Fixes 'Custom' being in list of Bluetooth Braille Displays, when there
are no Bluetooth Braille Displays (in new ChromeVox Settings Page [1]).
go/chromevox-prefs-and-page-migration

[1] chrome://os-settings/textToSpeech/chromeVox

Fixed: b/281743542
AX-Relnotes: N/A.
Change-Id: I938a81389cfea5936060604f90ab6c97c0be0669
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4519026
Reviewed-by: Wes Okuhara <wesokuhara@google.com>
Reviewed-by: Katie Dektar <katie@chromium.org>
Commit-Queue: Josiah Krutz <josiahk@google.com>
Cr-Commit-Position: refs/heads/main@{#1142724}