Allow missing type without converting to vector

Not sure if it makes sense, but, would it be possible to allow missing type without copying the underlining arrow vector? As far as I understand, allowing missing only changes the `Type` of arrow vector (e.g. in `Primitive`) not the underlying data.

The issue mentions Type, Primitive, and the underlying Arrow vector, but names no files, tests, or entry points. Start by locating the missing-type handling for Arrow vectors and determine whether the requested behavior can avoid copying the underlying vector. Done means missing type is allowed while the original vector remains un-copied.

Written by the indexing model from the issue text.