STYLE: simplify short-circuit involving autoPtr (#1775)

- with '&&' conditions, often better to check for non-null autoPtr
  first (it is cheap)

- check as bool instead of valid() method for cleaner code, especially
  when the wrapped item itself has a valid/empty or good.
  Also when handling multiple checks.

  Now
      if (ptr && ptr->valid())
      if (ptr1 || ptr2)

  instead
      if (ptr.valid() && ptr->valid())
      if (ptr1.valid() || ptr2.valid())

src/sampling/sampledSet/shortestPath/shortestPathSet.C

@@ -969,7 +969,7 @@ void Foam::shortestPathSet::genSamples
             markLeakPath,
             iter,
             mesh,
-            (isBlockedFace.valid() ? isBlockedFace() : isBoundaryFace),
+            (isBlockedFace ? *isBlockedFace : isBoundaryFace),
             insidePoint,
             insideCelli,
             outsidePoint,