RecordArray loses its field in a slice that doesn't select fields (v2)

[jpivarski]

Version of Awkward Array

HEAD

Description and code to reproduce

Here's a good slice:

>>> ak._v2.Array([[[1, 2, 3], [4, 5, 6]]])[:, []]
<Array [[[], []]] type='1 * var * var * int64'>

There's no record, no field being picked out, and it says, "all of the first dimension, none of the second." The result is correct.

Here's another good slice:

>>> ak._v2.Array([{"x": [[1, 2, 3], [4, 5, 6]]}])[:, [0]]
<Array [{x: [[1, 2, 3]]}] type='1 * {x: 1 * var * int64}'>

There's a record array, but a field is not picked out, and it says, "all of the first dimension, one item from the second." The result is correct.

But here's a bad slice:

>>> ak._v2.Array([{"x": [[1, 2, 3], [4, 5, 6]]}])[:, []]
<Array [{}] type='1 * {}'>

There's a record array, but a field is not picked out, and it says, "all of the first dimension, none of the second." The result is a record array with no fields: that's wrong—there should still be an x field, even though it will be empty.

Somewhere, an ak._v2.contents.RecordArray is not being populated correctly.