Allow empty bsStyle attribute

Consider the following example:

```js
var MyComponent = React.createClass({
  validationState: function(property) {
    if (this.state.validationMessages[property]) return 'error';
    return '';
  },
  render: function() {
    return (
       // A lot of components here
       <Input bsStyle={this.validationState('foo')} hasFeedback help={this.state.validationMessages.foo} type="text" value={this.state.something} />
     );
  }
});
```

this way you decorate your form inputs _only_ for error states. This already works as expected, it just logs a warning to console.