arch-arm: Avoid creating an empty byteEnable vector
arch-arm: Avoid creating an empty byteEnable vector

This behaviour will be forbidden in following patches.
Instead, create an all true vector.

JIRA: https://gem5.atlassian.net/browse/GEM5-196

Change-Id: I61d2852610281f2d7c7a669dcb4d2728be194f52
Signed-off-by: Giacomo Travaglini <8eef13a4f970e2e9372c8d26ad0737b760f99c02@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23524
Reviewed-by: Jason Lowe-Power <68c46a606457643eab92053c1c05574abb26f861@lowepower.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5