function getSortComparator(fnCompare) {
		return function(vValue1, vValue2) {
			if (vValue1 === vValue2) {
				return 0;
			}
			if (vValue1 === null) {
				return -1;
			}
			if (vValue2 === null) {
				return 1;
			}
			return fnCompare(vValue1, vValue2);
		};
	}
Creates a comparator usable for sorting.

The OData comparators return "NaN" for comparisons containing null values. While this is a
valid result when used for filtering, for sorting the null values need to be put in order, so
the comparator must return either -1 or 1 instead, to have null sorted at the top in
ascending order and on the bottom in descending order.

@param {function} fnCompare Function to compare two values with
@returns {function} The sort comparator

@private