[FIX] web_editor: fontawesome subnodes should make a node non-empty

For each node that the `insert` command inserts, the previous node is evaluated
and tested to be a `shrunk` block. If it is, it is considered "invisible" and
removed.

Prior to this commit, an element containing sub-elements with a fontawesome icon
was considered to be `isEmptyBlock==='true'` even though it is supposedly
visible as it contains that icon.

Therefore in cases where multiple nodes are inserted at once, if one of them has
only fa-icon nodes as children, that node would not have been inserted.

Task-3284580

closes odoo/odoo#119792

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>