Prevent schema from accepting null values for keyword
Prevent schema from accepting null values for keyword

keyword should always be required. This was an oversight in the update
on #269

Chuck8080/opendata,fedspendingtransparency/fedspendingtransparency.github.io,fedspendingtransparency/fedspendingtransparency.github.io,fedspendingtransparency/fedspendingtransparency.github.io,judecalvillo/project-open-data.github.io,pjdufour/project-open-data.github.io,Chuck8080/opendata,cmurphyUSAID/implementation-guide,judecalvillo/project-open-data.github.io,bpushed/project-open-data.github.io,pjdufour/project-open-data.github.io,pjdufour/project-open-data.github.io,portofsandiego/project-open-data.github.io,fedspendingtransparency/fedspendingtransparency.github.io,portofsandiego/project-open-data.github.io,bpushed/project-open-data.github.io,portofsandiego/project-open-data.github.io,pjdufour/project-open-data.github.io,portofsandiego/project-open-data.github.io