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ARROW-8929: [C++] Set the default for compute::Arity::VarArgs to 0

As Micah pointed out, 0 is a more reasonable default for the minimum number of arguments than 1 is.

Closes #7322 from wesm/ARROW-8929

Authored-by: Wes McKinney <wesm+git@apache.org>
Signed-off-by: Wes McKinney <wesm+git@apache.org>