Email: Fix null string label on empty subject

Steps to reproduce:

1.- Send a message from a email account that doesn't have signature (a pop3 for example) to any other
email account in the device (the same account or other pop3/imap/exchange)
2.- Receive the message
3.- Respond or forward the message
4.- The subject of the message will be Re: null or Fwd: null

Change-Id: I063001916412b13665212f5108d56dfc62639435
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>