test-calendarspec: do not convert timezone "" to ":"

I *think* it doesn't actually make any difference, because ":" will be ignored.
437f48a471 added prefixing with ":", but didn't
take into account the fact that we also use "" with a different meaning than
NULL here. But let's restore the original behaviour of specifying the empty
string.

src/test/test-calendarspec.c

@@ -44,7 +44,7 @@ static void _test_next(int line, const char *input, const char *new_tz, usec_t a
         if (old_tz)
                 old_tz = strdupa(old_tz);
 
-        if (new_tz)
+        if (!isempty(new_tz))
                 new_tz = strjoina(":", new_tz);
 
         assert_se(set_unset_env("TZ", new_tz, true) == 0);