ClientModel: Use BinaryData.Empty instead of bespoke solution

[annelo-msft]

See PR comment: #41016 (comment)

This will involve:

• Upgrading to System.Memory.Data 6.0.0 or higher
• Upgrading to System.Text.Json 6.0.0, which System.Memory.Data brings as a dependency
• Checking with PowerShell and Functions to ensure we don't upgrade above a version that they're using

[m-redding]

This should be unblocked by #46134