Simplify checking of container (List/HashTable, strings) sizes

- can use 'XX.empty()' instead of 'XX.size() == 0', 'XX.size() < 1' or
  'XX.size() <= 0' or for simpler coding.
  It also has the same number of characters as '!XX.size()' and /might/ be
  more readable

- many size checking had 'XX.size() > 0', 'XX.size() != 0', or 'XX.size() >= 1'
  when a simple 'XX.size()' suffices

src/meshTools/octree/octree.C

@@ -621,12 +621,12 @@ void Foam::octree<Type>::printStats(Ostream& os) const
         << "  nLeaves  :" << nLeaves() << endl
         << "  nEntries :" << nEntries() << endl;
 
-    if (nLeaves() > 0 && shapes().size() > 0)
+    if (nLeaves() && shapes().size())
     {
         os
             << "  Cells per leaf :"
             << scalar(nEntries())/nLeaves()
-            << endl
+            << nl
             << "  Every cell in  :"
             << scalar(nEntries())/shapes().size() << " cubes"
             << endl;