[feature/template-engine] Fix negative variable expressions

compile_tag_if had the flawed approach of adding an isset statement for
all variables to the beginning of the if. This fails for negative
expressions, and checking those takes a considerable effort.

The easier solution is to make the variable expression itself
conditional, defaulting to null if it is not set.

Thanks to naderman for the solution.

PHPBB3-9726

tests/template/templates/basic.html

@@ -16,5 +16,8 @@ fail
 <!-- BEGINELSE -->
 pass
 <!-- END empty -->
+<!-- IF not S_EMPTY -->
+pass
+<!-- ENDIF -->
 
 <!-- DUMMY var -->