runtime: null is also false; important for dealing with json

(cherry picked from commit 837c9d39abc6e45c202fca2f050b5b9210720b11)
(close #170)

src/erlydtl_runtime.erl

@@ -250,6 +250,7 @@ are_equal(_, _) ->
 is_false("") -> true;
 is_false(false) -> true;
 is_false(undefined) -> true;
+is_false(null) -> true;
 is_false(0) -> true;
 is_false("0") -> true;
 is_false(<<"0">>) -> true;