Fix NullPointerException when Integer deserializer returns null

This pull requests reproduces and fixes a NullPointerException when using GuavaOptional with a deserialized value of null.
The added unit test in the pull request simulates my use case in which data is read from a CSV file to an ObjectNode that contains strings, and from there it is read into an Object that assigns the string to Optional<Integer> field. The Integer deserializer returns null, and the GuavaOptional deserializer should convert that in Optional.absent(). Instead a NPE is thrown.