STYLE: avoid copy operation in fileName::isAbsolute()

src/OpenFOAM/primitives/strings/fileName/fileName.C

@@ -56,8 +56,7 @@ Foam::fileName::Type Foam::fileName::type() const
 
 bool Foam::fileName::isAbsolute() const
 {
-    fileName fName(*this);
-    return fName.size() && fName.operator[](0) == '/';
+    return !empty() && operator[](0) == '/';
 }