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/CascadingParametersDialog.java
private String formatPattern;
	private String formatCategroy;
	private boolean loading = true;
	private Button isRequired;
	private int maxStrLengthProperty;
	private int maxStrLengthOption;
	private String PROPERTY_LABEL_STRING[] = {
			LABEL_PARAM_NAME,
			LABEL_GROUP_PROMPT_TEXT,
			LABEL_PARAMTER_PROMPT_TEXT,
			LABEL_DATA_TYPE,
			LABEL_DISPLAY_TYPE,
			LABEL_DEFAULT_VALUE
	};
	private String OPTION_LABEL_STRING[] = {
			LABEL_HELP_TEXT, LABEL_FORMAT_AS, LABEL_LIST_LIMIT
	};
	protected int getMaxStrLength( String string[], Control control )
	{
		int len = UIUtil.getMaxStringWidth( string, control );
		return len;
	}
	/**
	 * 
	 * Constructor.
	 * 
	 * @param parentShell
	 * @param title
	 */