[PORT] [OAuth]do not set signInLink to empty when OAuthAppCredentials is set

[tomlm]

Port this change from botbuilder-dotnet/master branch:
microsoft/botbuilder-dotnet#3564

When OAuthAppCredentials is passed in as part OAuthPromptSettings, do not set SignInLink to be empty as Emulator/other first party channels will ignore this and get a SignInLink without the additional OAuthAppCredentials setting.

Changed projects

• Microsoft.Bot.Builder.Dialogs

[axelsrz]

This was solved in #863