[Slider] Fix "TypeError: Cannot read 'getBoundingClientRect' of null"

[noratarano]

I couldn't repro this error, but I think this should fix it.

TypeError: null is not an object (evaluating 'this.handleContainerNode.getBoundingClientRect')
  at getSliderBoundingBox (./node_modules/rheostat/lib/Slider.js:325)
  at None ([native code])
  at touch (./node_modules/rheostat/lib/Slider.js:704)
  at None ([native code])
  at this (./node_modules/rheostat/lib/Slider.js:677)
...
(2 additional frame(s) were not displayed)

By protecting access to this.handleContainerNode, we avoid this issue completely.

@ljharb @lencioni

Airbnb internal ticket: PRODUCT-49741

[ljharb]

I think you could add some unit tests for all the places that return early with getSliderBoundingBox returns falsy. (the condition inside getSliderBoundingBox shouldn't happen since the set-ref method is constructor-bound :-/)