fix: avoid panic if offset index not exists.

[RinChanNOWWW]

Which issue does this PR close?

Closes #.

Rationale for this change

index_reader::read_pages_locations will return an empty Vec<PageLocation> if there isn't an offset index:

pub fn read_pages_locations<R: ChunkReader>(
    reader: &R,
    chunks: &[ColumnChunkMetaData],
) -> Result<Vec<Vec<PageLocation>>, ParquetError> {
    let fetch = chunks
        .iter()
        .fold(None, |range, c| acc_range(range, c.offset_index_range()));

    let fetch = match fetch {
        Some(r) => r,
        None => return Ok(vec![]),
    };

    let bytes = reader.get_bytes(fetch.start as _, fetch.end - fetch.start)?;
    let get = |r: Range<usize>| &bytes[(r.start - fetch.start)..(r.end - fetch.start)];

    chunks
        .iter()
        .map(|c| match c.offset_index_range() {
            Some(r) => decode_offset_index(get(r)),
            None => Err(general_err!("missing offset index")),
        })
        .collect()
}

However, it will be set as a Some(ParquetOffsetIndex) to ParquetMetaData's offset_index.

When we read data by a ChunkReader, we will get page locations of a column by i[rg_idx][self.column_idx]. But i[rg_idx] maybe empty and it will lead to panic!

What changes are included in this PR?

Check if offset_index is empty before getting PageLocations from it.

Are there any user-facing changes?

No.

[tustvold]

Could we get a test for this please?

[RinChanNOWWW]

Could we get a test for this please?

Done. PTAL.

[tustvold]

Thank you