Use proper NULLs in the database for missing user_agent and referrer.
Use proper NULLs in the database for missing user_agent and referrer.


Former-commit-id: bc6cf473b894f8c5fd9c27df38b75f6cc6913862
eggpi/citationhunt,eggpi/citationhunt,guilherme-pg/citationhunt,eggpi/citationhunt,guilherme-pg/citationhunt,guilherme-pg/citationhunt,guilherme-pg/citationhunt,eggpi/citationhunt