Updated last4 to return masked value if empty in theme
Updated last4 to return masked value if empty in theme

closes https://github.com/TryGhost/Team/issues/464

Currently, if the last4 value for a customer is empty, we end up showing `null` in theme where the card details are shown. The fix updates the last4 to return a masked value of **** instead of null if it's empty, ensuring themes using the last4 values don't have a `null` value shown to the user.

TryGhost/Ghost,bitjson/Ghost,bitjson/Ghost,TryGhost/Ghost,ErisDS/Ghost,janvt/Ghost,ErisDS/Ghost,janvt/Ghost,e10/Ghost,e10/Ghost,ErisDS/Ghost,TryGhost/Ghost