Allow 0 to be a valid value

If I set `value` to 0, it would get rendered as null:
```
 echo Slider::widget([
            'name' => 'somevalue',
            'value'=> 0,
            'pluginOptions' => [
                'min' => 0,
                'max' => 15,
                'step' => .25
            ],
        ]);
```


```
var slider_bf093871 = {"min":0,"max":15,"step":0.25,"value":null,"id":"w0-slider"};
```
And the slider would get positioned at some random value.

With my patch, it renders correctly:
```
var slider_449938cc = {"min":0,"max":15,"step":0.25,"value":0,"id":"w0-slider"};
```