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/triSurface/booleanOps/surfaceIntersection/edgeIntersections.C

@@ -255,7 +255,7 @@ bool Foam::edgeIntersections::inlinePerturb
 
     const labelList& edgeEnds = classification_[edgeI];
 
-    if (edgeEnds.size() > 0)
+    if (edgeEnds.size())
     {
         bool perturbStart = false;
         bool perturbEnd = false;
@@ -701,7 +701,7 @@ Foam::label Foam::edgeIntersections::removeDegenerates
         // Transfer and test.
         edgesToTest.transfer(newEdgesToTest);
 
-        if (edgesToTest.size() == 0)
+        if (edgesToTest.empty())
         {
             FatalErrorIn("perturb") << "oops" << abort(FatalError);
         }

src/meshTools/triSurface/booleanOps/surfaceIntersection/surfaceIntersection.C

@@ -1044,7 +1044,7 @@ Foam::surfaceIntersection::surfaceIntersection
     transfer(allCutPoints, cutPoints_);
 
     // Shortcut. 
-    if ((cutPoints_.size() == 0) && (cutEdges_.size() == 0))
+    if (cutPoints_.empty() && cutEdges_.empty())
     {
         if (debug)
         {

src/meshTools/triSurface/booleanOps/surfaceIntersection/surfaceIntersectionFuncs.C

@@ -291,7 +291,7 @@ void Foam::surfaceIntersection::writeIntersectedEdges
     {
         const labelList& extraVerts = edgeCutVerts[edgeI];
 
-        if (extraVerts.size() != 0)
+        if (extraVerts.size())
         {
             const edge& e = surf.edges()[edgeI];