[config-service] return empty list, not None
[config-service] return empty list, not None

projects.get_refs docstring says that it returns a mapping where value is a list
but the it might return None instead of the list. Return an empty list instead.

Bug:
Change-Id: I67b5ab8c24701922779c134d19acc6592d6da721
Reviewed-on: https://chromium-review.googlesource.com/775562
Reviewed-by: Mun Yong Jang <cafb0219fa074fe0c089777a22a0dd4cc2126d7e@google.com>
Commit-Queue: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py