DRILL-7476: Set lastSet on TransferPair copies
Variable-width nullable vectors maintain a "lastSet" field
in the mutator. This field is used in "fill empties" logic
when setting the vector's value count. This is true even
if the vector is read-only, or has been transferred from
another (read-only) vector. LastSet must be set to the
row count or the code will helpfully overwrite existing
offsets with 0.
closes #1922
