title: React `value` prop on `input` should not be null
body: <p>I'm new to react and I got this <code>input</code> in my component:</p>
  
  <pre><code>&lt;input
          type="text"
          placeholder="..."
          className="form-control"
          value={this.state.value}
          disabled={this.props.edit}
          readOnly={this.props.readOnly}
          onChange={(e) =&gt; this.state.value = e}
      /&gt;
  </code></pre>
  
  <p>Now if <code>this.state.value</code> is empty I get this warning: </p>
  
  <pre><code>Warning: `value` prop on `input` should not be null. Consider using an empty string to clear the component or `undefined` for uncontrolled components.
  </code></pre>
  
  <p>But if I change my input to:</p>
  
  <pre><code>&lt;input
          type="text"
          placeholder="..."
          className="form-control"
          value={this.state.value || ""}
          disabled={this.props.edit}
          readOnly={this.props.readOnly}
          onChange={(e) =&gt; this.state.value = e}
      /&gt;
  </code></pre>
  
  <p>I can't edit it anymore.</p>