1) Do not overwrite lines description if the line was not modified

Fixes #78833

2) Correctly recompute prices on quantity change.

When no new line was added to the SO through the matrix, the prices were 
not recomputed, even if the qty was modified for some products.

The prices have to be recomputed in this case since pricelist provide 
rules based on minimum qty.

Part-of: odoo/odoo#78902