Undefined error when attempting to call `getElementTopPosition`

[kylekellogg]

Based on this error I get when running GitBook 3.2.2:

Uncaught TypeError: Cannot read property 'top' of undefined
    at a (theme.js:formatted:5276)
    at HTMLLIElement.<anonymous> (theme.js:formatted:5315)
    at Function.each (theme.js:formatted:784)
    at de.fn.init.each (theme.js:formatted:694)
    at HTMLDivElement.l (theme.js:formatted:5313)
    at HTMLDivElement.dispatch (theme.js:formatted:2468)
    at HTMLDivElement.m.handle (theme.js:formatted:2400)

I've tracked it down to be from an optimized version of the while loop in getElementTopPosition of src/js/theme/navigation.js where $parent (and therefore $el) could be null. Accordingly, in PR #51 I've done the following:

1. A null check to the while loop of getElementTopPosition
2. An empty jQuery element check to the while loop of getElementTopPosition
3. Sensible defaults should the call to $el.position().top in getElementTopPosition fail