BagState.isEmpty needs a tighter spec

[kennknowles]

Consider the following:

BagState<X> myBag = // empty
ReadableState<Boolean> isMyBagEmpty = myBag.isEmpty();
myBag.add(bizzle);
bool
empty = isMyBagEmpty.read();

Should empty be true or false? We need a consistent answer, across all kinds of state, when snapshots are required.

Imported from Jira BEAM-2980. Original Jira may contain additional context.
Reported by: kenn.