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GH-36912: [Java] JDBC driver stops consuming roots if it sees an empty root (#38590)

### Rationale for this change
The JDBC driver for Flight SQL incorrectly stops iterating on a FlightStream if it encounters an empty root.

### What changes are included in this PR?
The driver now waits until FlightStream#next() returns false to determine if a FlightStream is fully-consumed instead of an empty root.

### Are these changes tested?
Yes, new tests have been written.

### Are there any user-facing changes?
No
* Closes: #36912

Authored-by: James Duong <james.duong@improving.com>
Signed-off-by: David Li <li.davidm96@gmail.com>