REF: `styler.to_latex` siunitx changes: render {} only when needed.
adds item on #41649 for better backwards compat, and this is probably slightly better anyway.

When:

 - `siunitx=True` columns headers are now rendered like `{} & {A} & {B} \\`
 - `siunitx=False` (default) then they are now rendered like ` & A & B \\` 

Rndering column headers with or without curly braces has no effect in the LaTeX output, except when using `siunitx` where they are required for it to work properly.
