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
set( Calendar.MILLISECOND, 0 );
			set( Calendar.SECOND, 0 );
			set( Calendar.MINUTE, 0 );
			set( Calendar.HOUR, 0 );
			set( Calendar.AM_PM, AM );
			set( Calendar.DATE, 1 );
		}
		else if ( iUnit == WEEK_OF_YEAR )
		{
			set( Calendar.MILLISECOND, 0 );
			set( Calendar.SECOND, 0 );
			set( Calendar.MINUTE, 0 );
			set( Calendar.HOUR, 0 );
			set( Calendar.AM_PM, AM );
			
			
			int weekDay = get( DAY_OF_WEEK );
			add( DATE, 1 - weekDay );
		}
		else if ( iUnit == DATE )
		{
			set( Calendar.MILLISECOND, 0 );
			set( Calendar.SECOND, 0 );
			set( Calendar.MINUTE, 0 );
			set( Calendar.HOUR, 0 );
			
			set( Calendar.AM_PM, AM );
		}
		else if ( iUnit == HOUR )
		{