Prevent schema from accepting null values for keyword

keyword should always be required. This was an oversight in the update
on #269
Improve JSON schema for distribution, keywords, PrimaryITInvestmentUII
Adds accessURL and format as possible fields outside of distribution
Fixed some rules to prevent empty arrays/strings
Relaxing requirement for keyword uniqueness as per #288
accessURL and format can be null outside of a distribution
Merge pull request #286 from philipashlock/json-schema-requirements-fix

JSON schema corrections