BagState.isEmpty needs a tighter spec

Consider the following:

```

BagState myBag = // empty
ReadableState 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](https://issues.apache.org/jira/browse/BEAM-2980). Original Jira may contain additional context.
Reported by: kenn.

Start with the BagState.isEmpty() and ReadableState example, then review the imported BEAM-2980 Jira issue for the missing context. Define a consistent specification for the returned value across state types, including when snapshots are required.

Written by the indexing model from the issue text.