arrow

GH-34667: [C++][Parquet] Test DeltaLengthByteArrayDecoder with invalid inputs (#34668)

### Rationale for this change

We should make sure that we are defensive when it comes to invalid inputs. Otherwise malicious actors might send invalid Parquet files as a way to crash a users service.

### What changes are included in this PR?

Makes sure we reject empty pages by deferring to the DeltaBitPackDecoder to error when the lengths header is missing. Also adds a DCHECK that validates `num_valid_values_` was initialized (it wasn't when `len = 0`). Also adds a few more general tests cases for the encoding, covering empty arrays and empty strings.

### Are these changes tested?

Yes, several unit tests have been added.

### Are there any user-facing changes?

**This PR contains a "Critical Fix".**
* Closes: #34667

Lead-authored-by: Will Jones <willjones127@gmail.com>
Co-authored-by: Rok Mihevc <rok@mihevc.org>
Signed-off-by: Will Jones <willjones127@gmail.com>