STYLE: ensure that emptyLabelList is known from List.H

- treat as a List constant without requiring inclusion of ListOps.H

- replace use of emptyList<label>() with emptyLabelList directly.
  The emptyList<T>() casting is disallowed with many modern compilers
  and now marked as deprecated (expect early removal).

- relocate labelList typedef to List.H for more general access.
  Similar reasoning to having labelUList defined in UList.H