Always clear class declaration buffer after adding declarations
Insert a safety check for null strings

This should be avoided by the data model implementation, but there seem
to be some issues with this under yet-to-be-identified conditions in the
JSON dumps.
Include method for registering new site information in Sites

This is useful to add missing sites information after loading the sites
table (which can happen if the sites table dump is older than the data
dump).
Document assumptions on site link URLs
Add some default values to JSON implementations
Use modern entity timer and file paths

Improved compatibility with code in ExampleHelpers.
Document bug fixes
Merge pull request #100 from Wikidata/rdf-export-fixes

Rdf export fixes