[Bug] BE can not start with config validator

**Describe the bug**

BE starts with error in be.out

```
validate cumulative_compaction_policy= failed
error read custom config file.
```

**To Reproduce**
Start a brand new BE process with latest code.

**Reason**

This is because in #5379, we add a BE config validator.
But the logic is incorrect when reading config from `be_custom.conf`
