MultiWriteBagOStuff: require nonempty 'caches' param

MultiWriteBagOStuff: require nonempty 'caches' param

MultiWriteBagOStuff::__construct() barfs if the 'caches' parameter is unset,
but it permits it to be an empty array. This means that subsequent operations
that need to reference the first (primary) cache backend need to check that
it is there at all.

Since there is not much sense in having a MultiWriteBagOStuff object with zero
cache backends, make the constructor require a nonempty array for 'caches', and
remove the subsequent checks that are made redundant by this new restriction.

Change-Id: I30f3e0dcbfe67570a368e64b8233cc0ba7f90b2f