init: Allow matching empty property values

When we have a property match along with an event trigger, we
currently don't allow matching empty property values, in other words,
properties that are unset.  For example, the below trigger would never
be run:

on zygote-start && property:persist.sys.fuse=""

That doesn't make sense though, it should be possible to match an
empty property value, so this change allows that trigger to match when
persist.sys.fuse is either empty or not set.

This continues to not match a '*' to an empty property, so

on zygote-start && property:persist.sys.fuse=*

will not run if persist.sys.fuse is empty or unset.

Test: the above triggers run appropriately
Change-Id: Ia57de7b96ad352590d0c82ff4ae95060b7361976