[FIX] web_editor: properly paste text in non-HTML fields

This resolves an issue with pasting in text only mode (no HTML): the
list of nodes to paste was being modified with the `_mergeAdjacentULs`
method which is irrelevant to text nodes. Because of that, a corrupted
list of nodes was inserted into the DOM.

opw-1962743

closes odoo/odoo#32454

Signed-off-by: Pierre Paridans <pparidans@users.noreply.github.com>