DeltaBitPackEncoder Pads Miniblock BitWidths With Arbitrary Values

[tustvold]

Describe the bug

https://github.com/apache/arrow-rs/blob/master/parquet/src/encodings/encoding.rs#L577 skips over the miniblock bit widths, and then only goes back and writes a value for the miniblocks that contain a non-zero number of values. The empty miniblocks are left with whatever value happens to be in the encoder's buffer.

To Reproduce

This is one of the underlying bugs behind apache/datafusion#1976

Expected behavior

Whilst the specification technically allows for arbitrary padding, it seems like a good idea to avoid non-deterministic output where possible