[C++] ExecuteScalarExpression, when calling a nullary function on a nullary batch, resets the batch length to 1

[asfimport]

At the moment ARROW-16286 prevents us from using ExecuteScalarExpression on nullary functions. However, if we bypass constant folding, then we run into another problem. The batch passed to the function always has length = 1.

This appears to be tied up with the logic of ExecBatchIterator that I don't quite follow entirely. However, we should be preserving the batch length of the input to ExecuteScalarExpression and passing that to the function.

Reporter: Weston Pace / @westonpace

_{Note: This issue was originally created as ARROW-16290. Please see the migration documentation for further details.}

[github-actions]

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