[fix](backup) fix show snapshot cauing mysql connection lost (#21520)

If this is no `info file` in repository, the mysql connection may lost when user executing `show snapshot on repo`,
```
2023-07-05 09:22:48,689 WARN (mysql-nio-pool-0|199) [ReadListener.lambda$handleEvent$0():60] Exception happened in one session(org.apache.doris.qe.ConnectContext@730797c1).
java.io.IOException: Error happened when receiving packet.
    at org.apache.doris.qe.ConnectProcessor.processOnce(ConnectProcessor.java:691) ~[doris-fe.jar:1.2-SNAPSHOT]
    at org.apache.doris.mysql.ReadListener.lambda$handleEvent$0(ReadListener.java:52) ~[doris-fe.jar:1.2-SNAPSHOT]
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) ~[?:1.8.0_322]
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) ~[?:1.8.0_322]
    at java.lang.Thread.run(Thread.java:750) ~[?:1.8.0_322]
```

This is because there are some field missing in returned result set.

fe/fe-core/src/main/java/org/apache/doris/backup/Repository.java

@@ -703,6 +703,10 @@ private List<String> getSnapshotInfo(String snapshotName, String timestamp) {
                     info.add(snapshotName);
                     info.add(Joiner.on("\n").join(tmp));
                     info.add("OK");
+                } else {
+                    info.add(snapshotName);
+                    info.add(FeConstants.null_string);
+                    info.add("ERROR: No info file found");
                 }
             }
         } else {