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/primitives/strings/fileName/fileName.C

@@ -238,9 +238,9 @@ void Foam::fileName::operator=(const char* str)
 
 Foam::fileName Foam::operator/(const string& a, const string& b)
 {
-    if (a.size() > 0)       // First string non-null
+    if (a.size())           // First string non-null
     {
-        if (b.size() > 0)   // Second string non-null
+        if (b.size())       // Second string non-null
         {
             return fileName(a + '/' + b);
         }
@@ -251,7 +251,7 @@ Foam::fileName Foam::operator/(const string& a, const string& b)
     }
     else                    // First string null
     {
-        if (b.size() > 0)   // Second string non-null
+        if (b.size())       // Second string non-null
         {
             return b;
         }

src/OpenFOAM/primitives/strings/word/wordI.H

@@ -162,17 +162,17 @@ inline void Foam::word::operator=(const char* q)
 
 inline Foam::word Foam::operator&(const word& a, const word& b)
 {
-    if (!b.size())
-    {
-        return a;
-    }
-    else
+    if (b.size())
     {
         string ub = b;
         ub.string::operator[](0) = char(toupper(ub.string::operator[](0)));
 
         return a + ub;
     }
+    else
+    {
+        return a;
+    }
 }
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

src/OpenFOAM/primitives/strings/word/wordIO.C

@@ -61,7 +61,7 @@ Foam::Istream& Foam::operator>>(Istream& is, word& w)
         string::stripInvalid<word>(w);
 
         // flag empty strings and bad chars as an error
-        if (!w.size() || w.size() != t.stringToken().size())
+        if (w.empty() || w.size() != t.stringToken().size())
         {
             is.setBad();
             FatalIOErrorIn("operator>>(Istream&, word&)", is)