Extend `ui:emptyValue` and complement it with `ui:initialValue` and `ui:required`

Prerequisites

• I have read the documentation

What theme are you using?

utils

Is your feature request related to a problem? Please describe.

When the JSON Schema is shared across forms, comes from an API, or is auto-generated, you can't easily set per-form defaults or change which fields are required without touching the schema itself.

Example: a "person details" schema has a country field. A US-only form wants to hide it and pre-fill "US". Right now you'd have to modify the schema or stuff the value into formData, which mixes up user input with form setup.

Describe the solution you'd like

Two new uiSchema properties:

ui:initialValue: Pre-fills a field on initial render and after reset. Takes priority over schema.default.

This is about what the user sees, while the existing ui:emptyValue is about what gets stored when a field is blank. They're complementary:

• ui:initialValue pre-fills the field on render and reset, keeping it from being blank
• ui:emptyValue is stored in formData whenever the field is blank, whether from initial state, user clearing, or reset

This means ui:emptyValue would be extended from its current scope (only kicks in when a user clears a widget) to kick in whenever a field is blank, regardless of why.

ui:required: Overrides the schema's required for a field, but only on the UI side.

• true: makes a non-required field required (validation + asterisk)
• false: hides the required asterisk on a schema-required field

Importantly, ui:required only affects the form-level required check and the UI indicator. It does not change the JSON Schema itself, so schema-level validation still runs as-is. If the schema says a field is required and it's left empty, validation will still fail regardless of ui:required: false. The intended use for false is when the field is guaranteed to have a value through initialValue or emptyValue.

const uiSchema = {
  country: {
    "ui:widget": "hidden",
    "ui:initialValue": "US",
    "ui:required": false,  // hidden + pre-filled, no need to show as required
  },
  nickname: {
    "ui:required": true,   // required in this form, even if the schema doesn't say so
  },
};

Worth noting: ui:required: false on a schema-required field without initialValue or emptyValue is misleading, since the UI says "optional" but validation still fails. A console.warn for this would be nice, similar to the existing warning for deprecated ui:widget object syntax.

Describe alternatives you've considered

ui:emptyValue partly covers the defaults use case, but only on the data side. It doesn't pre-fill the field for the user and doesn't help with overriding required.