[fenix] Fix errors in top_sites_count metric

We discovered in the Fenix metrics errors query [1] that
top_sites_count had a spike in the number of errors. It
seems to be recording a count of 0, which is an invalid value.

[1] https://sql.telemetry.mozilla.org/queries/67107/source#169983

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

@@ -553,8 +553,11 @@ class GleanMetricsService(private val context: Context) : MetricsService {
             adjustAdGroup.set(context.settings().adjustAdGroup)
             adjustCreative.set(context.settings().adjustCreative)
             adjustNetwork.set(context.settings().adjustNetwork)
-            hasTopSites.set(context.settings().topSitesSize > 0)
-            topSitesCount.add(context.settings().topSitesSize)
+            val topSitesSize = context.settings().topSitesSize
+            hasTopSites.set(topSitesSize > 0)
+            if (topSitesSize > 0) {
+                topSitesCount.add(topSitesSize)
+            }
 
             toolbarPosition.set(
                 if (context.settings().shouldUseBottomToolbar) {