Prefer isEmpty() for collections

This PR replaces the following conditions when checking for the size of a collection:

- `size() > 0` -> `!isEmpty()`
- `size() == 0` -> `isEmpty()`
- `size() != 0` -> `!isEmpty()`
- `Iterables.size(numbers) > 0` -> `Iterables.isEmpty(numbers)`

For collections that use `Iterables.isEmpty(somecollection)`, should we consider using `somecollection.isEmpty()`?