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/fvMotionSolver/fvMotionSolvers/displacement/interpolation/displacementInterpolationFvMotionSolver.C

@@ -362,7 +362,7 @@ Foam::displacementInterpolationFvMotionSolver::curPoints() const
     vectorField zoneDisp(displacements_.size(), vector::zero);
     forAll(zoneDisp, zoneI)
     {
-        if (times_[zoneI].size() > 0)
+        if (times_[zoneI].size())
         {
             zoneDisp[zoneI] = interpolateXY
             (

src/fvMotionSolver/motionDiffusivity/inverseDistance/inverseDistanceDiffusivity.C

@@ -75,7 +75,7 @@ Foam::tmp<Foam::scalarField> Foam::inverseDistanceDiffusivity::y() const
 
     labelHashSet patchSet(mesh.boundaryMesh().patchSet(patchNames_));
 
-    if (patchSet.size() > 0)
+    if (patchSet.size())
     {
         return tmp<scalarField>
         (

src/fvMotionSolver/pointPatchFields/derived/surfaceSlipDisplacement/surfaceSlipDisplacementPointPatchVectorField.C

@@ -195,7 +195,7 @@ void surfaceSlipDisplacementPointPatchVectorField::evaluate
     // Get fixed points (bit of a hack)
     const pointZone* zonePtr = NULL;
 
-    if (frozenPointsZone_.size() > 0)
+    if (frozenPointsZone_.size())
     {
         const pointZoneMesh& pZones = mesh.pointZones();