fix: ComboBox component resetting bound object to null when custom content is entered

[bianwenbo]

🐛 Bug Report

💻 Repro or Code Sample

• Use a ComboBox component with an IEnumerable as the data source.
• Select an existing value from the data source (works as expected).
• Enter custom content in the ComboBox (UI displays correctly), notice that the bound object with using @bind-Value in the backend is reset to null.

🤔 Expected Behavior

The bound object should retain the custom content entered in the ComboBox when @bind-Value is used.

😯 Current Behavior

When custom content is entered in the ComboBox, the bound object with @bind-Value is reset to null .

💁 Possible Solution

The ComboBox component may need to be updated to handle custom content correctly.

🔦 Context

This issue affects the functionality of the ComboBox component when dealing with custom content and data binding.

🌍 Your Environment

• OS & Device: [e.g. Windows 11, PC]
• Browser [e.g. Edge ]
• .NET and FAST Version [e.g. .NET 8]