wfe: don't render account contact when empty (#257)
wfe: don't render account contact when empty (#257)

From https://tools.ietf.org/html/rfc8555#section-7.1.2:

  (optional, array of string)

This commit fixes Pebble rendering "contact: null" in WFE
responses.
letsencrypt/pebble,letsencrypt/pebble,letsencrypt/pebble