A rule of thumb from UX perpective is that is your options are less then 5, use a radiobutton list. This would also solve your problem with the empty value, because in a radiobutton list the empty value wont show up! You can set the radiobutton list in a vertical orientation from the properties of the widget.
If you assign the enumeration as a type to an attribute and you set a validation rule on the object for the same attribute, making it required, then you won't anymore be able to save the object having choosen the empty value.
Also you can give the empty option a placeholder text, “Choose one”, but you can still select it though :-(
Also you can set the property of the dropdown-page-element to required showing a nice message when you select the empty value, but you can still select it though :-(
From AppStore: Check out the AutoComplete widget, it might do the job and maybe the DropdowndivConverter (based on bootstrap).