Handling of data that doesn't contain triples

I created an issue about how to handle data that has no triples such
as empty graphs and properties with no values.  If possible it would
be nice to formalize the processing behavior in the specs and tests.
However, I can see this issue crossing between philosophical semantic
web ideals and pragmatic programming behavior and usability.  Check
out the playground examples and see if it matches what you think
should happen.

https://github.com/json-ld/json-ld.org/issues/65

* []
  * http://json-ld.org/playground/?json-ld=%5B%5D&frame=%7B%7D
* {}
  * http://json-ld.org/playground/?json-ld=%7B%7D&frame=%7B%7D
* {"ex":[]}
  * http://json-ld.org/playground/?json-ld=%7B%22ex%22%3A%5B%5D%7D&frame=%7B%7D
* {"ex":{}}
  * http://json-ld.org/playground/?json-ld=%7B%22ex%22%3A%7B%7D%7D&frame=%7B%7D

-dave