Remove "N/A" fallback values from contact information

[jampukka]

Related issue #69

Currently if user omits following properties from configuration:

api.contact.name=
api.contact.email=
api.contact.url=

hakunapi will use N/A as fallback value for each property, which is never valid value for email or url.

Instead just omit the contact information from the OpenAPI description altogether if none of the properties is set and don't set non-null values for fields that are not configured.

Additionally change the finnish_addresses example to reflect the change.