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

applications/utilities/postProcessing/dataConversion/foamToVTK/readFields.C

@@ -58,7 +58,7 @@ void readFields
         ++iter
     )
     {
-        if (!selectedFields.size() || selectedFields.found(iter()->name()))
+        if (selectedFields.empty() || selectedFields.found(iter()->name()))
         {
             fields.set
             (