test(formatter_htmlish): rename nul fixture directory for Windows (#28522)

`crates/oxc_formatter_htmlish/tests/fixtures/html/nul/` (added in
#28272) uses `nul`, a reserved device name on Windows, which breaks all
Windows jobs on main:

- `Test Windows`: `error: invalid path
'crates/oxc_formatter_htmlish/tests/fixtures/html/nul/nul-as-written.html'`
- `Test NAPI * (windows)`: `Could not find a part of the path '\\.\nul'`
while downloading the repo as a local action

Renamed the directory to `nul-byte/` and updated the `.gitattributes`
entry and the `DIVERGENCES.md` pin. The snapshot content is unchanged.

`cargo test -p oxc_formatter_htmlish` passes
(`fixtures::html::nul_byte::nul_as_written` included).

This PR was created with AI assistance (Claude Code).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

.gitattributes

@@ -8,7 +8,7 @@ crates/oxc_formatter_graphql/tests/fixtures/graphql/crlf/** text=auto eol=crlf
 crates/oxc_formatter_htmlish/tests/fixtures/** text=auto eol=lf
 crates/oxc_formatter_htmlish/tests/fixtures/html/crlf/** text=auto eol=crlf
 # NUL bytes would make git read these as binary
-crates/oxc_formatter_htmlish/tests/fixtures/html/nul/** text eol=lf diff
+crates/oxc_formatter_htmlish/tests/fixtures/html/nul-byte/** text eol=lf diff
 crates/oxc_formatter_yaml/tests/fixtures/** text=auto eol=lf
 crates/oxc_formatter_yaml/tests/fixtures/yaml/crlf/** text=auto eol=crlf
 crates/oxc_formatter_markdown/tests/fixtures/** text=auto eol=lf