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ARROW-12679: [Java] JDBC->Arrow for NOT NULL columns.

Prior to this patch, the VectorSchemaRoot's schema coming out of the JDBC adaptor always has all columns as nullable, even when the SQL column is NOT NULL. Even if this has no immediate impact on performance, it throws away information that can be useful downstream.

The fix just replaces the `true` (for nullable) by the actual `isNullableColumn` information.

Closes #10285 from jmgpeeters/ARROW-12679

Authored-by: Joris Peeters <joris.mg.peeters@gmail.com>
Signed-off-by: liyafan82 <fan_li_ya@foxmail.com>