use present? to fail on empty strings
use present? to fail on empty strings

Sometimes the `source_url` for a cookbook is an empty string, not a
null. The `source_repo` metric will then return the empty string. So
check for presence instead of nilness.

Signed-off-by: Robb Kidd <11cd4f9ba93acb67080ee4dad8d03929f9e64bf2@chef.io>

chef/supermarket,robbkidd/supermarket,juliandunn/supermarket,robbkidd/supermarket,juliandunn/supermarket,chef/supermarket,robbkidd/supermarket,chef/supermarket,robbkidd/supermarket,juliandunn/supermarket,chef/supermarket,juliandunn/supermarket,chef/supermarket