fix "build: Only set the default verity key as a fallback"

PRODUCT_VERITY_SIGNING_KEY is already set elsewhere
(build/core/product.mk), although it's blank, which causes the ?=
check to fail, which kills the build since the variable won't ever be
set. Use `strip` to fix the check.

Signed-off-by: shagbag913 <sh4gbag913@gmail.com>
Change-Id: I005377754a1f035b79111174f05ae687ba83b188