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ARROW-14305: [C++][Compute] Fixing Valgrind errors in hash join node tests

On verifying contents of the Boolean column output from hash join test, Valgrind reported uninitialized data access.
I added clearing of an output bit vector for Boolean column before decoding.
This seems to have fixed Valgrind errors.

Closes #11395 from michalursa/ARROW-14305-hash-join-valgrind

Authored-by: michalursa <michal@ursacomputing.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>