discovery.php: don't exit(5) without new devices discovered (#8893)

Don't exit with a non-zero value in case no new devices were discovered, as non-zero codes are considered failed jobs for most cron implementations and produce logs like the following (in Debian):

```
cron.info CRON[963890]: (CRON) error (grandchild #963892 failed with exit status 5)
``` 

DO NOT DELETE THIS TEXT

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discovery.php

@@ -153,7 +153,7 @@ if (!isset($options['q'])) {
 
 logfile($string);
 
-if ($discovered_devices == 0) {
+if ($doing !== 'new' && $discovered_devices == 0) {
     # No discoverable devices, either down or disabled
     exit(5);
 }