[milo] return a special value if property not found
[milo] return a special value if property not found

Build.PropertyValue signature says it returns interface{}, but if property is
not found it returns "". Return a special value instead.

Bug:
Change-Id: I94b6ab86c074ab2ed410cab98c52c7c457d023b2
Reviewed-on: https://chromium-review.googlesource.com/816040
Reviewed-by: Ryan Tseng <2190fe9a39b4920ec7bd59c848ec5cccd163056f@chromium.org>
Commit-Queue: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go