Merge "Allow dtb image to be empty in vendor_boot image" am: eb05d5f2ab am: ef54a13462 am: bb55dd786b

Original change: https://android-review.googlesource.com/c/platform/system/tools/mkbootimg/+/2018598

Change-Id: Ia27cda5d5a304dc5b9bf763c9e56b0404ee308e8