Align Mantine’s behavior with other themes when clearing string fields

[mshibanami]

Reasons for making this change

Mantine and other themes behave differently in this scenario:

• GIVEN the user enters this JSON Schema to Playground > BLANK

  {
    "type": "object",
    "properties": {
      "value": {
        "type": "string"
      }
    }
  }

• WHEN the user types something into the value field and then clears it
• THEN the following values are set to formData:
  • Mantine: { "value": "" }
  • Other themes: {}

Problem Demo:

Screen.Recording.2025-11-17.at.6.20.04.PM_converted.mov

This pull request updates Mantine's behavior to match the other themes.

Here is a demo of my change:

Screen.Recording.2025-11-17.at.6.06.31.PM_converted.mov

This changes the following components:

• BaseInputTemplate
• PasswordWidget
• TextareaWidget

　

• No related tickets. (Sorry for creating this pull request directly without prior discussion!)
• No synchronous review required.

Checklist

• I'm updating documentation
  • I've checked the rendering of the Markdown text I've added
• I'm adding or updating code
  • I've added and/or updated tests. I've run npx nx run-many --target=build --exclude=@rjsf/docs && npm run test:update to update snapshots, if needed.
  • I've updated docs if needed
  • I've updated the changelog with a description of the PR
• I'm adding a new feature
  • I've updated the playground with an example use of the feature