[opt](stats) No more sync unknown stats since cannot serialize (#22775)

Gson can't serialize INFINITY under current configuration

fe/fe-core/src/main/java/org/apache/doris/statistics/StatisticsCache.java

@@ -241,6 +241,9 @@ public class StatisticsCache {
         final StatisticsCacheKey k =
                 new StatisticsCacheKey(tableId, idxId, colName);
         final ColumnStatistic c = ColumnStatistic.fromResultRow(columnResults);
+        if (c == ColumnStatistic.UNKNOWN) {
+            return;
+        }
         putCache(k, c);
         TUpdateFollowerStatsCacheRequest updateFollowerStatsCacheRequest = new TUpdateFollowerStatsCacheRequest();
         updateFollowerStatsCacheRequest.key = GsonUtils.GSON.toJson(k);