VS2017 repository listing spams the log with NRE when registry key is not found

[shana]

Looks like our 2017 codepath for listing repositories from the registry is not handling things right if the key doesn't exist (even though it probably doesn't crash, it spams the log with exceptions).

Error loading the repository list from the registry 'System.NullReferenceException: Object reference not set to an instance of an object.
   at GitHub.TeamFoundation.RegistryHelper.PokeTheRegistryForRepositoryList() in Y:\jenkins\workspace\GitHub for Visual Studio speakeasy jenkinskeep\src\GitHub.TeamFoundation.15\RegistryHelper.cs:line 26
   at GitHub.Services.VSGitServices.GetKnownRepositories() in Y:\jenkins\workspace\GitHub for Visual Studio speakeasy jenkinskeep\src\GitHub.TeamFoundation.14\Services\VSGitServices.cs:line 118'

[shana]

From #948