Allow language maps to have a `null` value, which is ignored. Related to json-ld/json-ld.org#375.
Allow language maps to have a `null` value, which is ignored.
Related to json-ld/json-ld.org#375.

ruby-rdf/json-ld,PerfectMemory/json-ld,ruby-rdf/json-ld,PerfectMemory/json-ld,gkellogg/json-ld