Silence warnings about empty if/else bodies in release builds.

As debug_print expands to nothing unless PADWALKER_DEBUGGING is
set gcc will warn about "suggest braces around empty body in an
‘if’ statement" (same for ´else´) if ´-Wempty-body´ is used.
And that one is implied by ´-W´.

By defining the macro to a no-op statement we provide a
suitable replacement rather than nothing. A common technique
also used for e.g. assert().