Bug 153523 OK button in parameter dialog is enabled without default value in combo-box
Description: OK button in parameter dialog is enabled without default value in combo-box Steps to reproduce: 1. New a report parameter, combo-box. 2. Click New button, input abc as value, click OK 3. Set abc as default value and close Edit Parameter dialog. 4. New a data source and dataset 5. Edit the parameter, remove abc value and import values from dataset Expected result: OK button is disabled for there is no default value. Actual result: OK button is enabled.
UI/org.eclipse.birt.report.designer.ui/src/org/eclipse/birt/report/designer/ui/dialogs/ParameterDialog.java
* 
	 * @param value
	 *            the value to check
	 * @return Returns the error message if the input value is invalid,or null
	 *         if it is valid
	 */
	private String isValidValue( String value )
	{
		if ( canBeNull( ) )
		{
			if ( value == null || value.length( ) == 0 )
			{
				return null;
			}
		}
		else
		{
			if ( value == null || value.length( ) == 0 )
			{
				return ERROR_MSG_CANNOT_BE_NULL;
			}
		}
		if ( canBeBlank( ) )
		{
			if ( StringUtil.isBlank( value ) )
			{
				return null;
			}
		}
		else