Bug 202254 Exception is thrown when category values are null and type of X axis is set to DateTime
chart/org.eclipse.birt.chart.engine/src/org/eclipse/birt/chart/util/CDateTime.java
{
	private static final long serialVersionUID = 1L;
	private static final int MILLIS_IN_SECOND = 1000;
	private static final int MILLIS_IN_MINUTE = MILLIS_IN_SECOND * 60;