arrow

GH-36318: [Go] only decode lengths for the number of existing values, not for all nvalues. (#36322)

### Rationale for this change

Fixes issue 36318.
DeltaLengthBinaryArray Encoding fails to handle null values.

### What changes are included in this PR?

Instead of decoding lengths for "all" values (even the undefined ones), we only decode the lengths for the actually set values.
The Go Version of arrow was unable to read parquet files it produced itself if the mentioned encoding was used but the values contain nulls.

### Are these changes tested?

Tests are included.

### Are there any user-facing changes?

No.

**This PR contains a "Critical Fix".**
* Closes: #36318

Authored-by: Justin Heesemann <jheesemann@argo.ai>
Signed-off-by: Matt Topol <zotthewizard@gmail.com>