Subtitle not rendered as expected when its values is zero

### Problem
When the value of the **Subtitle** is equal to **zero** the size of the **Title** becomes bigger and the value of the **Subtitle** stays in front of the Title instead of below it.

### Screenshot


### Sugestion

In line 151 of the file [node-renderer-default.js](https://github1s.com/frontend-collective/react-sortable-tree/blob/HEAD/src/node-renderer-default.js) there is no verification if the variable `nodeSubtitle` is an integer, so the value zero becomes falsy.

##### Reference
Here is an [article](https://kentcdodds.com/blog/use-ternaries-rather-than-and-and-in-jsx) that may help to solve the problem.