Email Sent activity record should store HTML version of message if it's not empty rather than TEXT version

Currently we always store TEXT version of the sent email. However, since the default state of the Send Email form presents user with HTML message input - this approach is wrong (when viewing the resulting activity, the message body will appear to be empty unless the user also entered a TEXT version of their message).

When inserting the Sent Email activity record, we should check for non-empty HTML message and store that in civicrm_activity.details if present. NOTE: non-empty means it has content other than spaces, line and paragraph breaks.