Fix crash when trying to view empty auth token list

[HebaruSan]

Problem

If you have never added any auth tokens (and therefore don't have the registry key HKEY_CURRENT_USER\Software\CKAN\AuthTokens), you may receive a null reference exception if you:

• Open the GUI settings window
• Edit the auth tokens in consoleui
• Run ckan authtoken list

This can be worked around by adding a fake auth token, which you may have to do from the command line with ckan authtoken add host token.

Cause

Win32Registry.GetAuthTokenHosts() returns null because Microsoft.Win32.Registry.CurrentUser.OpenSubKey returns null.

Changes

Now Win32Registry.GetAuthTokenHosts() returns a zero-element string array if Microsoft.Win32.Registry.CurrentUser.OpenSubKey returns null.

Fixes #2300.

[politas]

Strange. It worked fine when I added my authtoken.