Fix issue where inputs flip between controlled and uncontrolled...

Fixes the following warning:

```
Warning: FriggingBootstrap.Input is changing a uncontrolled input
of type text to be controlled. Input elements should not switch from
uncontrolled to controlled (or vice versa). Decide between using a
controlled or uncontrolled input element for the lifetime of the
component.
```

frig-js/frigging-bootstrap#44 (comment)

src/components/input.js

@@ -113,7 +113,7 @@ export default class Input extends React.Component {
           this.context.frigForm.errors[this.props.name] || []
         )}
         saved={this.context.frigForm.saved[this.props.name]}
-        value={this.context.frigForm.data[this.props.name]}
+        value={this.context.frigForm.data[this.props.name] || ''}
         onChange={this._onChange}
       />
     )