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

src/OpenFOAM/algorithms/dynamicIndexedOctree/dynamicIndexedOctree.C

@@ -2398,7 +2398,7 @@ const Foam::labelList& Foam::dynamicIndexedOctree<Type>::findIndices
         return *(contents_[getContent(contentIndex)]);
     }
 
-    return emptyList<label>();
+    return Foam::emptyLabelList;
 }
 
 

src/OpenFOAM/algorithms/indexedOctree/indexedOctree.C

@@ -2660,7 +2660,7 @@ const Foam::labelList& Foam::indexedOctree<Type>::findIndices
 {
     if (nodes_.empty())
     {
-        return emptyList<label>();
+        return Foam::emptyLabelList;
     }
 
     labelBits index = findNode(0, sample);
@@ -2675,7 +2675,7 @@ const Foam::labelList& Foam::indexedOctree<Type>::findIndices
         return contents_[getContent(contentIndex)];
     }
 
-    return emptyList<label>();
+    return Foam::emptyLabelList;
 }