Dropdown’s `displayValue` returns an empty string when having `type="combobox" multiple` attributes

[marchbox]

Seems the (this.isCombobox && this.multiple) in https://github.com/microsoft/fluentui/blob/master/packages/web-components/src/dropdown/dropdown.base.ts#L145 is a bug, because the code after the if statement does try to generate the display value for multiple select combobox.