Handle empty dictionary pages in DictDecoder.

[sclmn]

Currently, the code panics which prevents handling parquet files with 0-length dictionary and null values in the column

[etseidl]

Thanks @sclmn. Do you have a reproducer for this? I tried running your added test without your changes, and the result is not a panic, but the expected Err. Is something else panicking because it doesn't handle the error correctly?

[sclmn]

Sorry, I didn't realize this got submitted.

The ideal fix is not to return an error. For a column chunk with just nulls a dictionary with 0 length seems fine.

[etseidl]

For a column chunk with just nulls a dictionary with 0 length seems fine.

But the Parquet spec mandates that at least the bitwidth is written to the data page. https://github.com/apache/parquet-format/blob/master/Encodings.md#dictionary-encoding-plain_dictionary--2-and-rle_dictionary--8

0 would be a valid bitwidth, followed by no encoded data.

[sclmn]

Thanks, that makes sense.

The C++ version allows this and some writers produce files like this.