.Net: Use empty string instead of null content in assistant message in Azure/OpenAI connectors

[dmytrostruk]

Motivation and Context

Resolves: #8072
Resolves: #8488

Azure Chat Completion with data service returns 400 (Bad Request) Invalid chat message detected: message content must be string response when assistant message in chat history doesn't contain content property or when this property is an array of content items. This case is applicable for function calling, where AI model returns set of tools to call instead of message content.

This PR contains a change to set content property as an empty string only for cases when content doesn't exist (null).

Description

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄