STYLE: check autoPtr as plain bool instead of valid()

- cleaner code, more similarity with unique_ptr

  Now
      if (ptr)
      if (!ptr)

  instead
      if (ptr.valid())
      if (!ptr.valid())

src/OpenFOAM/db/functionObjects/writeFile/writeFile.C

@@ -241,7 +241,7 @@ Foam::OFstream& Foam::functionObjects::writeFile::file()
         return Snull;
     }
 
-    if (!filePtr_.valid())
+    if (!filePtr_)
     {
         FatalErrorInFunction
             << "File pointer not allocated\n";