Add check for blank DKIM to support disabling email

[dotBATmanNO]

Add support / recommendation for adding a blank DKIM record to domains that are not used for e-mail
*._domainkey. TXT "v=DKIM1; p="

DKIM check should identify blank DKIM record to add to scoring capability.

See https://www.gov.uk/guidance/protect-domains-that-dont-send-email