Bug 120503 Cascading Parameter can not drop into layout
Details: Cascading Parameter can not drop into layout Step: 1.Open a report design and Create a Sample Data Source and data set by "select * from CLASSICMODELS.CUSTOMERS" 2.New a cascading parameter and create CUSTOMERSNUMBER and CUSTOMERSNAME in parameter named number and name. Press OK 3.Drop parameter name into layout. Actual results: The parameter in Cascading Parameter Group can not drop to layout
UI/org.eclipse.birt.report.designer.core/src/org/eclipse/birt/report/designer/util/DEUtil.java
* If given value if null, return an empty string, or return itself.
	 * 
	 * @param value
	 *            a String value.
	 * @return non-null value.
	 */
	public static String resolveNull( String value )
	{
		return value == null ? "" : value; 
	}
	/**
	 * Converts the RGB object value to a String, the String format is "r,g,b",
	 * no quotation marks.
	 * 
	 * @param rgb
	 *            RGB value.
	 * @return String value.
	 */
	public static String getRGBText( RGB rgb )
	{
		if ( rgb != null )
		{
			return rgb.red + "," + rgb.green + "," + rgb.blue; 
		}
		return ""; 
	}
	/**
	 * Converts the String value to an RGB object value, the String format is
	 * "r,g,b", no quotation marks.
	 *