keymaps: fix empty "right option" key.

Fixes #18833.

Change-Id: I5d46ea11581af48e6fd6f38003a589ca700b07f0
Reviewed-on: https://review.haiku-os.org/c/haiku/+/7497
Reviewed-by: nephele nephele <nep-git@packageloss.eu>
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>

Add standard TIS-820.2538 keymap for Thai keyboard.

Change-Id: I87196163a40d7e0bd67159d4dc3d7e4298f8acfd
Reviewed-on: https://review.haiku-os.org/641
Reviewed-by: Adrien Destugues <pulkomandy@pulkomandy.tk>