BotFrameworkClientImpl.PostActivityAsync() doesn't support null fromBotId and toBotId values
Version
4.13.1
Describe the bug
Anonymous Skills via null fromBotId and toBotId values was dropped in the BotFrameworkClientImpl.PostActivityAsync() method:
https://github.com/microsoft/botbuilder-dotnet/blob/57eeae891efe22c632d60907ca28533f1d86fbe5/libraries/Microsoft.Bot.Connector/Authentication/BotFrameworkClientImpl.cs#L39-L46
This is a regression based off of previous behavior in the BotFrameworkHttpClient.
Background
fromBotId and toBotId null values are supported in BotFrameworkHttpClient.PostActivityAsync() for anonymous bot scenarios.
https://github.com/microsoft/botbuilder-dotnet/blob/57eeae891efe22c632d60907ca28533f1d86fbe5/libraries/integration/Microsoft.Bot.Builder.Integration.AspNet.Core/BotFrameworkHttpClient.cs#L183-L186
https://github.com/microsoft/botbuilder-dotnet/blob/57eeae891efe22c632d60907ca28533f1d86fbe5/libraries/integration/Microsoft.Bot.Builder.Integration.AspNet.Core/BotFrameworkHttpClient.cs#L260-L265
Additional context
The BotFrameworkClientImpl class is used in the AdaptiveDialogBot, which is used in the Adaptive Runtime work and by extension, Composer 2.0.
However, this isn't a problem in Composer (tested w/ 2.0.0-nightly.241915.204a82) unless null values are provided for the Host's AppId or Skill's AppId in the config.
Relevant code in JS: https://github.com/microsoft/botbuilder-js/pull/3698/files#diff-612cd898ceba4f30c7d406690b0a5f370a4146194cf32f8b82d43aa93dbb4723R50-R51