Fix parse invalid interval string

[rchindris]

The function _parse_iso8601_interval raises IndexError when either the first or the last part of the interval is not specified. This fix checks if the interval parts are empty strings and raises ParserError if so.

Pull Request Check List

• [x] Added tests for changed code.
• [x] Updated documentation for changed code.

[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #860 will not alter performance

_{Comparing rchindris:master (c44ffb0) with master (b84b976)}

Summary

✅ 1 untouched benchmarks

[Secrus]

Please rebase your changes with master to fix the failing pipeline

[rchindris]

@Secrus 👍