branch-2.1: [fix](jdbc)make sure init the jdbc client before using it

[morningman]

This may cause some statistic collection task fail:

2025-06-05 15:02:32,277 WARN (Statistics Job Appender|182) [ExternalCatalog.buildDbForInit():902] Failed to check db DORIS_TEST exist in remote system, ignore it.
java.lang.NullPointerException: Cannot invoke "org.apache.doris.datasource.jdbc.client.JdbcClient.getDatabaseNameList()" because "this.jdbcClient" is null
        at org.apache.doris.datasource.jdbc.JdbcExternalCatalog.listDatabaseNames(JdbcExternalCatalog.java:267) ~[doris-fe.jar:1.2-SNAPSHOT]
        at org.apache.doris.datasource.ExternalCatalog.getFilteredDatabaseNames(ExternalCatalog.java:476) ~[doris-fe.jar:1.2-SNAPSHOT]
        at org.apache.doris.datasource.ExternalCatalog.lambda$makeSureInitialized$0(ExternalCatalog.java:318) ~[doris-fe.jar:1.2-SNAPSHOT]
        at com.github.benmanes.caffeine.cache.LocalLoadingCache.lambda$newMappingFunction$2(LocalLoadingCache.java:145) ~[hive-catalog-shade-2.1.4.jar:2.1.4]
        at com.github.benmanes.caffeine.cache.LocalCache.lambda$statsAware$0(LocalCache.java:139) ~[hive-catalog-shade-2.1.4.jar:2.1.4]
        at com.github.benmanes.caffeine.cache.BoundedLocalCache.lambda$doComputeIfAbsent$14(BoundedLocalCache.java:2406) ~[hive-catalog-shade-2.1.4.jar:2.1.4]
        at java.util.concurrent.ConcurrentHashMap.compute(ConcurrentHashMap.java:1916) ~[?:?]

Only for branch-2.1.
Fix master in #51471

[hello-stephen]

Thank you for your contribution to Apache Doris.
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4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[morningman]

run buildall

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