Empty nullable fields are crushed for unknown reason since last 6th in beta version

[adrndfn]

Prerequisites

• [x] I have searched the existing issues
• [x] I understand that providing a SSCCE example is tremendously useful to the maintainers.
• [x] I have read the documentation
• [ ] Ideally, I'm providing a sample JSFiddle, Codesandbox.io or preferably a shared playground link demonstrating the issue.

What theme are you using?

mui

Version

6.0.0-beta.10

Current Behavior

Empty nullable select are crushed for unknown reasons seems last versions

Expected Behavior

Expected behaviour should be input size depends of label when select nullable is empty (no selection)

Steps To Reproduce

1. Using updated mui theme and 6.0.0-beta.10 rjsf version

2. Set a schema with a nullable enum of string (and maybe with other simple type like nullable string to compare render )

        "example-of-enum": {
            "anyOf": [
                {
                    "type": "string",
                    "enum": [
                        "Test1",
                        "Test2",
                    ],
                    "ui:enumNames": [
                        "Test1",
                        "Test2",
                    ]
                },
                {
                    "type": "null",
                    "default": null
                }
            ],
            "title": "Example of enum"
        },
        "example-of-string": {
            "anyOf": [
                {
                    "type": "string"
                },
                {
                    "type": "null",
                    "default": null
                }
            ],
            "title": "Example of string"
        },

3. Use this schema in a uiSchema RJSForm page / react environment

4. Identify crushing problems on nullable empty select mui component ( created from previous string enum nullable ) in comparaison to text field for example.

Environment

- OS: debian
- Node: v22.13.1
- npm: 10.9.2

Anything else?

No response