daemon/watchdog: fix watchdog detection when $WATCHDOG_PID is unset

This fixes SdWatchdogEnabled() behavior when $WATCHDOG_PID is unset,
aligning it with the native method:
```
If the $WATCHDOG_USEC environment variable is set, and the $WATCHDOG_PID
variable is unset or set to the PID of the current process, the service
manager expects notifications from this process.
```

Author: lucab

daemon/watchdog.go

@@ -39,12 +39,13 @@ func SdWatchdogEnabled() (time.Duration, error) {
 		return 0, fmt.Errorf("error converting WATCHDOG_USEC: %s", err)
 	}
 	if s <= 0 {
-		return 0, fmt.Errorf("error WATCHDOG_USEC must a positive number")
+		return 0, fmt.Errorf("error WATCHDOG_USEC must be a positive number")
 	}
+	interval := time.Duration(s) * time.Microsecond
 
 	wpid := os.Getenv("WATCHDOG_PID")
 	if wpid == "" {
-		return 0, nil
+		return interval, nil
 	}
 	p, err := strconv.Atoi(wpid)
 	if err != nil {
@@ -54,5 +55,5 @@ func SdWatchdogEnabled() (time.Duration, error) {
 		return 0, nil
 	}
 
-	return time.Duration(s) * time.Microsecond, nil
+	return interval, nil
 }

daemon/watchdog_test.go

@@ -40,10 +40,10 @@ func TestSdWatchdogEnabled(t *testing.T) {
 		{"100", mypid, false, 100 * time.Microsecond},
 		{"50", mypid, false, 50 * time.Microsecond},
 		{"1", mypid, false, 1 * time.Microsecond},
+		{"1", "", false, 1 * time.Microsecond},
 
 		// No-op cases
 		{"", mypid, false, 0}, // WATCHDOG_USEC not set
-		{"1", "", false, 0},   // WATCHDOG_PID not set
 		{"1", "0", false, 0},  // WATCHDOG_PID doesn't match
 		{"", "", false, 0},    // Both not set