Add a graceful exit for AuthenticationManager::initializeConnection

Add a graceful exit for AuthenticationManager::initializeConnection
https://bugs.webkit.org/show_bug.cgi?id=186632
<rdar://problem/41041033>

Reviewed by Brent Fulgham.

Source/WebKit:

Add a graceful exit for AuthenticationManager::initializeConnection when the provided IPC connection
is null or the underlying xpc connection is null.

* Shared/Authentication/cocoa/AuthenticationManagerCocoa.mm:
(WebKit::AuthenticationManager::initializeConnection):

Source/WTF:

* wtf/spi/darwin/XPCSPI.h:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@232937 268f45cc-cd09-0410-ab3c-d52691b4dbfc