Fix resend after import

After importing a message via 'File' -> 'Open...', sending that
message via 'Message' -> 'Send again...' was broken, as the latter
menu action would simply do nothing.

Fix this issue by not returning from slotResendMessage() if message
viewer's associated Akonadi::Item is not valid, which is always the
case on imported MIME message files.

Additionally check whether there is a payload instead of checking
for a valid item ID for deciding whether to gray out 'Send Again...'.