fix(library/hop_match): Lua bindings gotcha

See http://www.luafaq.org/gotchas.html#T6.4

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>

src/library/hop_match.cpp

@@ -261,7 +261,7 @@ static int hop_match(lua_State * L) {
             if (s) {
                 push_expr(L, *s);
             } else {
-                lua_pushnil(L);
+                lua_pushboolean(L, false);
             }
             lua_rawseti(L, -2, i);
             i = i + 1;