Fix IndexError when splitting ANSI-only strings

[yousefissa]

ANSI-only strings such as a trailing color reset parse into a FmtStr with no chunks. Width-aware line splitting indexed the first chunk unconditionally, causing the IndexError reported in #194 when displaying colorized logs.

Return no lines for a chunkless FmtStr, matching plain empty-string behavior. Keep column validation unchanged. Regression tests cover reset-only and color-only input, empty construction, narrow columns, styled log wrapping, and preservation of zero-width combining text.

Fixes #194.

Validation:

• Full suite, including doctests and terminal tests, on Python 3.10.17 and 3.14.6: 113 passed, 1 skipped, 4 expected failures on each. Terminal runs used TERM=xterm-256color with NO_COLOR unset.
• Black checks and source/wheel builds pass.
• Type checking reports the same 28 existing errors in window.py as unchanged upstream; none in the modified file.

AI assistance was used to implement and validate this change.