Fix compilation warnings with GCC 6.1

The new GCC 6.1 considers that "this" is always different from NULL,
the compiler produces a warning, and the code is optimized in this
sens.

 this ? "something" : "somethingelse";

will always return "something", even if this is NULL.

This C++ trick was used with the alist

alist->size()    => if null, it returns 0
alist->empty()   => if null, it returns true