Use `.empty()` method instead of `.size() == 0`

dune/grid/uggrid/uggridfactory.cc

@@ -260,7 +260,7 @@ createGrid()
   typedef typename std::set<UGGridBoundarySegment<dimworld> >::iterator SetIterator;
 
   BoundaryExtractor::detectBoundarySegments(elementTypes_, elementVertices_, boundarySegments);
-  if (boundarySegments.size() == 0)
+  if (boundarySegments.empty())
     DUNE_THROW(GridError, "Couldn't extract grid boundary.");
 
   std::vector<int> isBoundaryNode;