Do not render empty mark labels

[uwolfer]

Use-case: I have a slider bar for percent values. I use marks for
every allowed value (e.g. values from 2 to 5 are not allowed).
Displaying every mark label would flood the UI. So there are mark labels
displayed only for every 10th step, which also works fine without this
change (using: {10: '10%', 11: ''}).
This change just optimizes that there are no empty DOM nodes rendered.

[coveralls]

Coverage remained the same at 61.739% when pulling 0f74224 on uwolfer:empty-mark-labels into 76d4cf3 on react-component:master.

[paranoidjk]

Sorry, could you explain what problems does render empty mark labels will cause ?

IMO, empty string actually is a valid React Element, if we add too many such check in a React Component, it will be redundancy.

[uwolfer]

It's not a bugfix. It's just an optimization. This change just prevents rendering useless empty dom nodes.