[Bug] Hit none partition cache, but hit range is still right

[xinghuayu007]

Describe the bug
Doris supports two kinds of cache mode: sql_cache and partition_cache. sql_cache takes sql string as key and cache the whole data. partition_cache splits the data into many partition data and caches them differently. Therefore a query may hit part of the partition_cache data. If a query hits the left part of the data, we call the hit range is left. If a query hits the right part of the data, we call the hit range is right. And if a query hits the whole part of the data, we call the hit range is full.

A query does not hit any partition cache, but the algorithm still returns hit range right.

To Reproduce
Steps to reproduce the behavior:

1. create a table:
   CREATE TABLE table_date (
   event_day date NULL COMMENT "",
   siteid int(11) NULL DEFAULT "10" COMMENT "",
   citycode smallint(6) NULL COMMENT "",
   username varchar(32) NULL DEFAULT "" COMMENT "",
   pv bigint(20) SUM NULL DEFAULT "0" COMMENT ""
   ) ENGINE=OLAP
   AGGREGATE KEY(event_day, siteid, citycode, username)
   COMMENT "OLAP"
   PARTITION BY RANGE(event_day)
   (PARTITION p202006 VALUES [('0000-01-01'), ('2020-07-01')),
   PARTITION p202007 VALUES [('2020-07-01'), ('2020-08-01')),
   PARTITION p202008 VALUES [('2020-08-01'), ('2020-09-01')),
   PARTITION p202009 VALUES [('2020-09-01'), ('2020-10-01')),
   PARTITION p202010 VALUES [('2020-10-01'), ('2020-11-01')))
   DISTRIBUTED BY HASH(siteid) BUCKETS 10
   PROPERTIES (
   "replication_num" = "1",
   "in_memory" = "false",
   "storage_format" = "V2"
   );
2. enable partition cache
   set enable_partition_cache = true;
3. add a log to function handleCacheStmt
LOG.warn("cache-mode:{}, hit-range:{}, cacheResul:{}", mode.name(), cacheAnalyzer.getHitRange(), cacheResult);
4. execute a query
   select count(*) from table_date where event_day >= "2020-06-02" and event_day <= "2020-10-11" group by event_day;
5. See error
   the log prints as follow:
   cache-mode:Partition, hit-range:Right, cacheResul:null

Expected behavior
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