do not facet null-valued properties; closes #69
do not facet null-valued properties; closes #69

facets are not created for null-valued properties in the following
sections:

- citation
- biological_context
- metadata

this change has the most affect on metadata, because the evidence
schema does not put constaints on null values.

OpenBEL/openbel-server,OpenBEL/openbel-api,nbargnesi/openbel-server,OpenBEL/openbel-api,nbargnesi/openbel-server,nbargnesi/openbel-server,OpenBEL/openbel-server,OpenBEL/openbel-api,OpenBEL/openbel-server,OpenBEL/openbel-api