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### _Why are the changes needed?_
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The previous default value of `kyuubi.engine.initialize.sql` and `kyuubi.engine.session.initialize.sql` both are `SHOW DATABASES`.
The intention of `SHOW DATABASES`is to eagerly active HiveClient, it's quite redundant for executing many times.
This PR proposes to change `kyuubi.engine.session.initialize.sql`'s default value to `Nil`
### _How was this patch tested?_
- [ ] Add some test cases that check the changes thoroughly including negative and positive cases if possible
- [ ] Add screenshots for manual tests if appropriate
- [x] [Run test](https://kyuubi.readthedocs.io/en/latest/develop_tools/testing.html#running-tests) locally before make a pull request
Closes#1248 from pan3793/sess-init.
Closes#1248
ad2bdd7d [Cheng Pan] Update doc
0736857c [Cheng Pan] Change kyuubi.engine.session.initialize.sql default value to Nil
Authored-by: Cheng Pan <chengpan@apache.org>
Signed-off-by: Cheng Pan <chengpan@apache.org>
8fba917673