Proactive C# sample needs to stop sending mock appId

[johnataylor]

The current proactive sample in C# sends a mock appId (its actually a new GUID)

This only ever worked because we had a check in the AppCredentials implementation that looked in the trusted service url collection. Now we have removed that check this samples was broken.

To do anonymous auth the sample should make the appId null.

See also:

https://github.com/microsoft/botbuilder-dotnet/pull/4891

[johnataylor]

@stevkan note the change we need to make to the sample when we have @EricDahlvang fix in a package.