[FIX] web_editor: prevent undefined class/style

The `_wrap` util function of `convert_inline` was not checking if
`className` and `style` were defined before adding them to the wrapper
element, leading to things like `<div class="undefined">element</div>`.

This is a backport of commit [1] from 16.0.

[1]: https://github.com/odoo/odoo/commit/ac8aa12125063aae6ef7357827f3763732440ae0

Part-of: odoo/odoo#125146