[select] Fix SelectValue placeholder not rendered with Record items

[vcode-sh]

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Summary

<Select.Value placeholder="..." /> renders an empty <span> when items is a Record<string, ReactNode> without a "null" key. This is caused by inverted logic in hasNullItemLabel().

The same function is shared by Combobox, so <Combobox.Value placeholder="..." /> with Record items is also affected.

Before (buggy):

// packages/react/src/utils/resolveValueLabel.tsx
if (!Array.isArray(items)) {
  return items != null && !('null' in items);
}

For { sans: "Sans-serif", serif: "Serif" }:

• !('null' in items) → true → returns true
• SelectValue skips the placeholder because it thinks the null value already has a label

After (fixed):

if (!Array.isArray(items)) {
  return items != null && 'null' in items;
}

Changes

• resolveValueLabel.tsx: Remove the ! operator in the Record branch of hasNullItemLabel()
• resolveValueLabel.test.ts: Add tests for Record items (with and without "null" key), flat array items, and undefined

Fixes #4136

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/@base-ui/react@4137
  

  pnpm add https://pkg.pr.new/@base-ui/utils@4137
  

commit: 181f00f

[mui-bot]

Bundle size report

Bundle	Parsed size	Gzip size
@base-ui/react	▼-3B(0.00%)	▼-1B(0.00%)

Details of bundle changes

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[atomiks]

Thanks for the fix @vcode-sh, can you add these to SelectValue.test.tsx too:

    it('displays placeholder when object items do not have a null key', async () => {
      const items = {
        option1: 'Option 1',
        option2: 'Option 2',
      };

      await render(
        <Select.Root items={items}>
          <Select.Value data-testid="value" placeholder="Select an option" />
        </Select.Root>,
      );

      expect(screen.getByTestId('value')).to.have.text('Select an option');
    });

    it('null key label in object items takes precedence over placeholder', async () => {
      const items = {
        null: 'None',
        option1: 'Option 1',
      };

      await render(
        <Select.Root items={items}>
          <Select.Value data-testid="value" placeholder="Select an option" />
        </Select.Root>,
      );

      expect(screen.getByTestId('value')).to.have.text('None');
    });

[vcode-sh]

Thanks @atomiks