Bug 182673 No default value error message prompts incorrectly
Steps to reproduce: 1. New a report parameter; 2. Choose control type as Combo; 3. Choose parameter as dynamic type; Expected result: As the isRequired is false by default, parameter's default value can be null value. There should be no error message about no default value prompt. Actual result; Error messages prompts
UI/org.eclipse.birt.report.designer.ui/src/org/eclipse/birt/report/designer/ui/dialogs/ParameterDialog.java
{
			if ( valueTable.getColumnProperties( ).length == 4
					&& columnIndex == 1 )
			{
				SelectionChoice choice = ( (SelectionChoice) element );
				if ( isDefaultChoice( choice ) )
				{
					return DEFAULT_ICON;
				}
			}
			return null;
		}
		public String getColumnText( Object element, int columnIndex )
		{
			SelectionChoice choice = ( (SelectionChoice) element );
			final int valueIndex = valueTable.getColumnProperties( ).length - 2;
			String text = null;
			if ( valueTable.getColumnProperties( ).length == 4
					&& columnIndex == 1 )
			{
				if ( isDefaultChoice( choice ) )
				{
					text = FLAG_DEFAULT;
				}
			}
			else if ( columnIndex == valueIndex )
			{
				text = choice.getValue( );
			}
			else if ( columnIndex == valueIndex + 1 )