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**Do you want to request a *feature* or report a *bug*?**

Bug.

**What is the current behavior?**

Creating an `<input type='submit'` with `value={undefined}` results in a button with no text. https://codesandbox.io/s/nn7p94y3rl

The same behaviour can be seen with `type='reset'`

I have a PR open with a proposed fix. https://github.com/facebook/react/pull/12780

**What is the expected behavior?**

The input should use the browser's default value for the input (Submit for en, etc.).

**Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?**

React 16.3.2. Chrome 66.0.3359.139. macOS 10.13.4. Works in React 15 (unsure of earlier versions)

ReactDOM.render(
          <input value={() => {}} onChange={() => {}} />,
          container,
        ),
      ).toWarnDev('Invalid value for prop `value`');
      const node = container.firstChild;
      expect(node.value).toBe('');
      expect(node.getAttribute('value')).toBe('');
    });
    it('treats updated function value as an empty string', function() {
      ReactDOM.render(<input value="foo" onChange={() => {}} />, container);
      expect(() =>
        ReactDOM.render(
          <input value={() => {}} onChange={() => {}} />,
          container,
        ),
      ).toWarnDev('Invalid value for prop `value`');
      const node = container.firstChild;
      expect(node.value).toBe('');
      expect(node.getAttribute('value')).toBe('');
    });
    it('treats initial function defaultValue as an empty string', function() {
      ReactDOM.render(<input defaultValue={() => {}} />, container);
      const node = container.firstChild;
      expect(node.value).toBe('');
      expect(node.getAttribute('value')).toBe('');
      
    });
    it('treats updated function defaultValue as an empty string', function() {
      ReactDOM.render(<input defaultValue="foo" />, container);