fix: ChunkString returns empty slice for empty input (fixes #788)

[nghiack7]

Summary

ChunkString("", n) was returning [""] (a one-element slice containing an empty string) instead of [] (an empty slice). This was inconsistent with Chunk([]rune{}, n) which correctly returns an empty slice.

Root Cause

The early-return guard if size >= len(str) { return []T{str} } always fires when str is empty because any positive size satisfies size >= 0. So it returned []T{""} instead of an empty slice.

Fix

Add an explicit early-return for the empty-string case before the size check:

if len(str) == 0 {
    return []T{}
}

Test

Updated the existing test case (which already carried a // @TODO: should be [] comment referencing #788) to assert the correct behavior.

// Before
ChunkString("", 2) // => [""]

// After  
ChunkString("", 2) // => []

Full test suite passes with no regressions.

Fixes #788

[nghiack7]

Friendly ping — this is a one-line fix for ChunkString("", n) returning [""] instead of []. All tests pass ✅. Fixes #788.