Narwhals v2.27.1

What's Changed

• fix: return a numeric dtype from str.len_chars on all-null object columns by @FBruzzesi in #4031
• fix: count characters, not bytes, in str.zfill on Polars < 2.0 by @FBruzzesi in #4040

Full Changelog: v2.27.0...v2.27.1