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/OpenFOAM/db/IOstreams/Fstreams/IFstream.C

@@ -42,7 +42,7 @@ Foam::IFstreamAllocator::IFstreamAllocator(const fileName& pathname)
     ifPtr_(NULL),
     compression_(IOstream::UNCOMPRESSED)
 {
-    if (!pathname.size())
+    if (pathname.empty())
     {
         if (IFstream::debug)
         {

src/OpenFOAM/db/IOstreams/Fstreams/OFstream.C

@@ -46,7 +46,7 @@ Foam::OFstreamAllocator::OFstreamAllocator
 :
     ofPtr_(NULL)
 {
-    if (!pathname.size())
+    if (pathname.empty())
     {
         if (OFstream::debug)
         {