.Net: Update GeminiChatMessageContent to support System.Text.Json deserialization when calledToolResult is null (#11622)

### Motivation and Context

Changes from
https://github.com/microsoft/semantic-kernel/pull/11236/files

### Description

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These notes will help understanding how your code works. Thanks! -->

### Contribution Checklist

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- [ ] The code builds clean without any errors or warnings
- [ ] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [ ] All unit tests pass, and I have added new tests where possible
- [ ] I didn't break anyone 😄

Author: markwallace-microsoft

dotnet/src/Connectors/Connectors.Google/Models/Gemini/GeminiChatMessageContent.cs

@@ -3,6 +3,7 @@
 using System.Collections.Generic;
 using System.Linq;
 using System.Text;
+using System.Text.Json.Serialization;
 using Microsoft.SemanticKernel.ChatCompletion;
 using Microsoft.SemanticKernel.Connectors.Google.Core;
 
@@ -13,6 +14,14 @@ namespace Microsoft.SemanticKernel.Connectors.Google;
 /// </summary>
 public sealed class GeminiChatMessageContent : ChatMessageContent
 {
+    /// <summary>
+    /// Creates a new instance of the <see cref="GeminiChatMessageContent"/> class
+    /// </summary>
+    [JsonConstructor]
+    public GeminiChatMessageContent()
+    {
+    }
+
     /// <summary>
     /// Initializes a new instance of the <see cref="GeminiChatMessageContent"/> class.
     /// </summary>