stitch: Only check invariants when one or more is specified
stitch: Only check invariants when one or more is specified

When running a stitch with many containers, the invariants will be
checked when compiling. This takes a long time and used to run even
when there were no invariants to actually check. This patch makes it
so that invariants are only ever checked when at least one is given.

kklin/kelda,mmussomele/quilt,quilt/quilt,tejasmanohar/quilt,PenguinToast/quilt,quilt/quilt,NetSys/quilt,quilt/quilt,PenguinToast/quilt,kayousterhout/quilt,kayousterhout/quilt,secant/quilt,kelda/kelda,kayousterhout/quilt,tejasmanohar/quilt,ejj/quilt-dev,kklin/quilt,stevenlin598/quilt,secant/quilt,LuiseV/quilt,stevenlin598/quilt,LuiseV/quilt,LuiseV/quilt,LuiseV/quilt,stevenlin598/quilt,stevenlin598/quilt,PenguinToast/quilt,PenguinToast/quilt,ejj/quilt-dev,kklin/quilt,tejasmanohar/quilt,kelda/kelda,secant/quilt,kelda/kelda,mmussomele/quilt,NetSys/quilt,kklin/kelda,secant/quilt,kelda/kelda,kayousterhout/quilt,tejasmanohar/quilt,NetSys/quilt,NetSys/quilt,mmussomele/quilt,kklin/kelda,NetSys/quilt,stevenlin598/quilt,mmussomele/quilt,PenguinToast/quilt,kklin/quilt,ejj/quilt-dev,stevenlin598/quilt,quilt/quilt,mmussomele/quilt,kklin/quilt,kklin/kelda,mmussomele/quilt,ejj/quilt-dev,secant/quilt,LuiseV/quilt