Do not format USEC_INFINITY as NULL

systemctl would print 'CPUQuotaPerSecUSec=(null)' for no limit. This
does not look right.

Since USEC_INFINITY is one of the valid values, format_timespan()
could return NULL, and we should wrap every use of it in strna() or
similar. But most callers didn't do that, and it seems more robust to
return a string ("infinity") that makes sense most of the time, even
if in some places the result will not be grammatically correct.

src/core/timer.c

@@ -242,7 +242,7 @@ static void timer_dump(Unit *u, FILE *f, const char *prefix) {
                                 "%s%s: %s\n",
                                 prefix,
                                 timer_base_to_string(v->base),
-                                strna(format_timespan(timespan1, sizeof(timespan1), v->value, 0)));
+                                format_timespan(timespan1, sizeof(timespan1), v->value, 0));
                 }
         }
 }