[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/template_test.php

@@ -91,7 +91,7 @@ class phpbb_template_template_test extends phpbb_test_case
 				array(),
 				array(),
 				array(),
-				"pass\npass\n<!-- DUMMY var -->",
+				"pass\npass\npass\n<!-- DUMMY var -->",
 			),
 			array(
 				'variable.html',