[Fix] fix multi database sync npe error

[JNSimba]

Proposed changes

Issue Number: close #xxx

Problem Summary:

When synchronizing multiple databases, not writing --database may cause null pointer errors

When synchronizing multiple databases, there are two scenarios

1. Synchronize multiple upstream databases to one database of doris

--database dorisdb
--mysql-conf database-name db.*

Note: Currently, if multiple databases have tables with the same name, similar to the scenario of sharding, they will be written to the same downstream table

2. Write multiple upstream databases to multiple downstream databases
   No need to specify --database
   Just specify --mysql-conf database-name db.*

Checklist(Required)

1. Does it affect the original behavior: (Yes/No/I Don't know)
2. Has unit tests been added: (Yes/No/No Need)
3. Has document been added or modified: (Yes/No/No Need)
4. Does it need to update dependencies: (Yes/No)
5. Are there any changes that cannot be rolled back: (Yes/No)

Further comments

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