[#5577] BotFrameworkClientImpl.PostActivityAsync() doesn't support null fromBotId and toBotId values

[ceciliaavila]

Fixes #5577

Description

This PR updates the PostActivityAsync method in BotFrameworkClientImpl to allow using null values for fromBotId and toBotId arguments. This will avoid failures when anonymous skill bots don't have the AppId property in their settings.

Specific Changes

• Replaced the throw errors by the assignation of an empty string as the default value in the method PostActivityAsync in BotFrameworkClientImpl.

Testing

The following images show Composer and SDK bots working with anonymous skills.

[coveralls]

Pull Request Test Coverage Report for Build 350780

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 4 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.0001%) to 79.093%

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Files with Coverage Reduction	New Missed Lines	%
/libraries/AdaptiveExpressions/BuiltinFunctions/GetNextViableTime.cs	1	90.91%
/libraries/AdaptiveExpressions/BuiltinFunctions/GetPreviousViableTime.cs	1	90.91%
/libraries/Microsoft.Bot.Streaming/Transport/WebSocket/WebSocketClient.cs	2	78.87%

Totals
Change from base Build 349053:	0.0001%
Covered Lines:	25831
Relevant Lines:	32659

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💛 - Coveralls

[BruceHaley]

✔️ No Binary Compatibility issues for Microsoft.Bot.Connector.dll