Bug 152430 Key can modify to empty string
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/api/SortKeyHandle.java
{
	/**
	 * Constructs the handle of sort key.
	 * 
	 * @param valueHandle
	 *            the value handle for sort key list of one property
	 * @param index
	 *            the position of this sort key in the list