[fenix] Fixes https://github.com/mozilla-mobile/fenix/issues/5653: Removes crash with null campaign

app/src/main/java/org/mozilla/fenix/components/metrics/AdjustMetricsService.kt

@@ -35,7 +35,9 @@ class AdjustMetricsService(private val application: Application) : MetricsServic
         )
 
         config.setOnAttributionChangedListener {
-            application.settings().adjustCampaignId = it.campaign
+            it.campaign?.let { campaign ->
+                application.settings().adjustCampaignId = campaign
+            }
         }
 
         config.setLogLevel(LogLevel.SUPRESS)