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/computation/DataSetIterator.java
{
				iContentType = IConstants.NON_PRIMITIVE_ARRAY;
				oa = (Object[]) oContent;
			}
			iDataType = IConstants.UNDEFINED;
		}
		if ( iContentType == IConstants.UNDEFINED )
		{
			throw new IllegalArgumentException( MessageFormat.format( Messages.getResourceBundle( )
					.getString( "exception.process.content.dataset" ), 
					new Object[]{
							oContent, ds
					} )
			);
		}
		if ( co != null )
		{
			it = co.iterator( );
		}
		iRowCount = getRowCountInternal( );
	}
	/**
	 * @return
	 */
	public final boolean isEmpty( )
	{
		return iRowCount <= 0;
	}
	/**
	 * @return