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
{
						String selectedValue = dialog.getSelectedValue( );
						if ( selectedValue != null )
							defaultValueChooser.setText( selectedValue );
					}
					else if ( status == Window.CANCEL )
					{
						defaultValueChooser.setText( "" );
					}
				}
				else
				{
					if ( selection.equals( CHOICE_NULL_VALUE ) )
					{
						changeDefaultValue( null );
					}
					else if ( selection.equals( CHOICE_BLANK_VALUE ) )
					{
						changeDefaultValue( "" );
					}
					if ( isStatic( ) )
					{
						refreshValueTable( );
					}
				}
			}
		} );
		defaultValueChooser.addModifyListener( new ModifyListener( ) {
			public void modifyText( ModifyEvent e )
			{