Make building an empty static library work.

Handle the case where a static library is not built because it has no
input files (e.g. because they are conditionally included and it happens
that the computed list is empty). Previously, ar never ran because of
split-long-arguments, and anything that depended on the static library
would fail to link. Make sure that ar runs at least once.

Change-Id: If0071d7ae50c4e45fff1772dca43187af155db1f