[c++] Use valid ASCII for wide-as-possible string current domain

[johnkerl]

Issue and/or context: As tracked on issue #2407 / [sc-51048].

Note that the intended Python and R API changes are all agreed on and finalized as described in #2407.

Changes:

Using "\xff" causes issues in TileDB-Py's tiledb.schema.dump, and is unnecessary regardless.

Notes for Reviewer:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.06%. Comparing base (58f92e5) to head (4a1e75e).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3367      +/-   ##
==========================================
+ Coverage   85.86%   86.06%   +0.20%     
==========================================
  Files          57       56       -1     
  Lines        6210     6116      -94     
==========================================
- Hits         5332     5264      -68     
+ Misses        878      852      -26     

Flag	Coverage Δ
python	86.06% <ø> (+0.20%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
python_api	86.06% <ø> (+0.20%)	⬆️
libtiledbsoma	∅ <ø> (∅)

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