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
}
	private boolean canBeNull( )
	{
		boolean canBeNull = false;
		if ( dirtyProperties.containsKey( CHECKBOX_ISREQUIRED ) )
		{
			canBeNull = !( ( (Boolean) dirtyProperties.get( CHECKBOX_ISREQUIRED ) ).booleanValue( ) );
		}
		else
		{
			canBeNull = !( inputParameter.isRequired( ) );
		}
		return canBeNull;
	}
	private boolean isDefaultChoice( SelectionChoice choice )
	{
		String choiceValue = choice.getValue( );
		String defaultValue = convertToStandardFormat( this.defaultValue );
		if ( canBeNull( ) && choiceValue == null && defaultValue == null )
		{
			return true;
		}
		return choiceValue != null && isEqual( choiceValue, defaultValue );
	}
	private boolean isStatic( )
	{
		return staticRadio.getSelection( );
	}
	private String convertToStandardFormat( String string )
	{