Represent '-1' indices as empty strings in StringArrayEncoding

[padix-key]

Fixes #831

Furthermore in BinaryCIF files

• the compression of short float arrays via compress() is improved,
• Decoded arrays from data encoded with only ByteEncoding is fixed.

To makes the CI pass, this PR also fixes a regex issue in afdb.fetch().

[codspeed-hq]

CodSpeed Performance Report

Merging #833 will not alter performance

_{Comparing padix-key:issue-831 (fce71c6) with main (f8b40d2)}

Summary

✅ 59 untouched