user registration fails if no default for 'tags' is set
user registration fails if no default for 'tags' is set

If 'tags' has no default value, the sql query for user registration will
silently fail.

Also see this thread regarding this issue:
https://community.openenergymonitor.org/t/emoncms-error-creating-user/6490/19

emoncms/emoncms,inverse/emoncms,inverse/emoncms,inverse/emoncms,chaveiro/emoncms,chaveiro/emoncms,chaveiro/emoncms,emoncms/emoncms,chaveiro/emoncms,emoncms/emoncms,inverse/emoncms,emoncms/emoncms,emoncms/emoncms