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>
TRDS:[UnifiedEmail] Dark Email (2/2)

Change-Id: Ib3b760fc4cb6bb346cb7e42bea856a238f065557
trds: color adjustments

Change-Id: Ifbb8853309c7a08958da5d89df252baf7ce1b0ee
trds: fix white bg on email compose for sw600dp

Fix it fix it :)

Change-Id: Ib4676869ae8e794442af040eb06b578e27042c5a