Fix issue with tk-desktop stomping on a specified authenticated user with the default user

[mbrainerd]

This fixes an issue where the previously authenticated user (initiated during tk-framework-desktopstartup's startup.py execution) is being overridden by tk-desktop setting the current user to CoreDefaultManager's default user. This bit us when attempting to have artists switch back and forth between multiple Shotgun servers, and the connection url being incorrect since it gets pulled from the default user rather than the authenticated user.

[unchris]

Would this fix this stack trace?

Traceback (most recent call last):
  File "/Users/ccameron/Library/Caches/Shotgun/desktop/install/app_store/tk-framework-desktopstartup/v1.10.0/python/shotgun_desktop/startup.py", line 720, in main
    settings
  File "/Users/ccameron/Library/Caches/Shotgun/desktop/install/app_store/tk-framework-desktopstartup/v1.10.0/python/shotgun_desktop/startup.py", line 334, in __launch_app
    return __post_bootstrap_engine(splash, app_bootstrap, engine, settings)
  File "/Users/ccameron/Library/Caches/Shotgun/desktop/install/app_store/tk-framework-desktopstartup/v1.10.0/python/shotgun_desktop/startup.py", line 488, in __post_bootstrap_engine
    startup_descriptor=startup_desc
  File "/Applications/Shotgun.app/Contents/Resources/Python/bundle_cache/app_store/tk-desktop/v2.4.12/python/tk_desktop/desktop_engine_site_implementation.py", line 467, in run
    self.desktop_window = desktop_window.DesktopWindow(console)
  File "/Applications/Shotgun.app/Contents/Resources/Python/bundle_cache/app_store/tk-desktop/v2.4.12/python/tk_desktop/desktop_window.py", line 170, in __init__
    "HumanUser", [["id", "is", user["id"]]],
TypeError: 'NoneType' object has no attribute '__getitem__'

It appears that user on line 170 is None after calling user = engine.get_current_login(). Based on the comments about it being about switching between accounts, my guess is this bit me because I had originally tried to sign into an incorrect URL, and then tried again with the correct one. Perhaps it then pulled None from the previous failed attempt to connect?