arrow

GH-40719: [Go] Make `arrow.Null` non-null for `arrow.TypeEqual` to work properly with `new(arrow.NullType)` (#40802)

### Rationale for this change

Currently creating a record with a `null` type via `new(arrow.NullType)` in the schema will fail the schema validation.

### What changes are included in this PR?

Made `arrow.Null` a non-null value instead of just a declaration.

### Are these changes tested?

Yes, see cd4253a24e6d828128fbb7854da3c37951d74885

### Are there any user-facing changes?

`arrow.Null` became non-null, but the type is the same.
* GitHub Issue: #40719

Authored-by: Alex Shcherbakov <candiduslynx@users.noreply.github.com>
Signed-off-by: Matt Topol <zotthewizard@gmail.com>