.Net: Align the behavior of the ChatMessageContent.Content property setter with streaming counterpart

[SergeyMenshykh]

Motivation and Context

This is a follow-up fix for the ChatMessageContent.Content property setter, similar to the one that was done to the StreamingChatMessageContent.Content property when addressing the PR comment: #6449 (comment).

Description

This change updates the text of the first text item regardless of the value being set. This helps avoid inconsistency that occurs when text content already exists in the items collection, null is set via the Content property, and the setter does nothing, while the getter returns the text message content:

var items = new ChatMessageContentItemCollection
{
    new TextContent("Hi AI."),
};

var chatMessage = new ChatMessageContent(AuthorRole.User, items: items);
chatMessage.Content = null; // The property setter does nothing if the value is null.

// Fails
Assert.Null(chatMessage.Content); // The property getter returns the "Hi AI." text.