router: Add default to HTTP route path column
router: Add default to HTTP route path column

This allows updates and backup restores to work. Before this change they
would fail with:

    ERROR: column "path" contains null values (SQLSTATE 23502)

Signed-off-by: Jonathan Rudenberg <3692bfa45759a67d83aedf0045f6cb635a966abf@titanous.com>
flynn/flynn,benbjohnson/flynn,lmars/flynn,flynn/flynn,lmars/flynn,benbjohnson/flynn,lmars/flynn,flynn/flynn,lmars/flynn,benbjohnson/flynn,philiplb/flynn,benbjohnson/flynn,philiplb/flynn,philiplb/flynn,philiplb/flynn,lmars/flynn,philiplb/flynn,benbjohnson/flynn,flynn/flynn,flynn/flynn,benbjohnson/flynn