Some properties are set as optional

Some properties that are designed to provide a safe default value are
explicitly set as optional using the 'a ?= b' syntax.

Bug: 117892318
Bug: 158735147
Test: m

Exempt-From-Owner-Approval: cherry-pick from master

Merged-In: Ie6a50ab7e0bcb210e282bc18e8c1daf412903f90
(cherry picked from commit dfb3937c)
Change-Id: Ie6a50ab7e0bcb210e282bc18e8c1daf412903f90