Self message causes crash if the signature is null

[Matthias Schoettle]

If, for whatever reason, a self message's signature is null, building the MessageCallView causes the following crash:

Caused by: java.lang.NullPointerException
    at ca.mcgill.sel.ram.ui.views.message.MessageCallView.isSuperCall(MessageCallView.java:500)
    at ca.mcgill.sel.ram.ui.views.message.MessageCallView.build(MessageCallView.java:239)
    at ca.mcgill.sel.ram.ui.views.message.MessageCallView.<init>(MessageCallView.java:173)
    at ca.mcgill.sel.ram.ui.views.message.MessageViewView.addMessageView(MessageViewView.java:559)
    at ca.mcgill.sel.ram.ui.views.message.MessageViewView.build(MessageViewView.java:339)

[Matthias Schoettle]

Resolves #502: Adds checks for message signature and lifeline represents to ensure they are not null.

Moves the signature to the from end to improve visibility for self messages.

→ <<cset a3268b3b9e29>>