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>

Author: candiduslynx

arrow/compare_test.go

@@ -42,6 +42,9 @@ func TestTypeEqual(t *testing.T) {
 		{
 			Null, Null, true, false,
 		},
+		{
+			Null, new(NullType), true, false,
+		},
 		{
 			&BinaryType{}, &StringType{}, false, false,
 		},

arrow/datatype_null.go

@@ -27,7 +27,5 @@ func (*NullType) Layout() DataTypeLayout {
 	return DataTypeLayout{Buffers: []BufferSpec{SpecAlwaysNull()}}
 }
 
-var (
-	Null *NullType
-	_    DataType = Null
-)
+// Null gives us both the compile-time assertion of DataType interface as well as serving a good element for use in schemas.
+var Null DataType = new(NullType)