[#6642] Handle null content

[ceciliaavila]

Fixes #6642

Description

This PR validates the request's content before using it to create the body of streaming requests.

Specific Changes

• Added validation of content before reading the data to create the request's body in CreateSteamingRequestAsync.

Testing

The following image shows the handle of the null content.

[coveralls]

Pull Request Test Coverage Report for Build 357955

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 3 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.001%) to 78.98%

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Files with Coverage Reduction	New Missed Lines	%
/libraries/AdaptiveExpressions/BuiltinFunctions/GetNextViableTime.cs	1	93.94%
/libraries/integration/Microsoft.Bot.Builder.Integration.AspNet.Core/CloudAdapter.cs	1	77.3%
/libraries/integration/Microsoft.Bot.Builder.Integration.AspNet.Core/ServiceCollectionExtensions.cs	1	93.55%

Totals
Change from base Build 356861:	0.001%
Covered Lines:	25937
Relevant Lines:	32840

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💛 - Coveralls

[BruceHaley]

✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.Integration.AspNet.Core.dll