fixed converters/valueToPosition (ptomasroos#190)

The fact that this function is still returning a position when value is undefined was causing me an issue in single slider mode as under some conditions (after changing min and max values) it was setting positionTwo equal to max.
I think it's better it returns undefined when value is undefined

Author: rainst

converters.js

@@ -39,6 +39,8 @@ const closest = (array, n) => {
 };
 
 export function valueToPosition(value, valuesArray, sliderLength) {
+  if (value === undefined)
+    return undefined;
   const index = closest(valuesArray, value);
 
   const arrLength = valuesArray.length - 1;