systemd: guard against currentIdentifiers being undefined

The initial state of `currentIdentifiers` is undefined, while unlikely
to be triggered as prependEntries() is called after some promises are
done.

Adding a link to the logs page from cockpit-podman triggered and
clicking it in our tests triggered:

```
TypeError: Cannot read properties of undefined (reading 'includes')"]
```

Author: jelly

pkg/systemd/logsJournal.jsx

@@ -168,7 +168,7 @@ export class JournalBox extends React.Component {
             const serviceTag = entries[i].SYSLOG_IDENTIFIER;
             this.renderer.prepend(entries[i]);
             // Only update if the service is not yet known
-            if (serviceTag && !this.props.currentIdentifiers.includes(serviceTag))
+            if (serviceTag && !(this.props.currentIdentifiers || []).includes(serviceTag))
                 // Due to asynchronous nature it needs to be checked whether this
                 // identifier is not yet defined. The previous check could be omitted
                 // and only this one used but let's try to trigger as few updates as possible