[FIX] stock: error when importing done qty w/o providing reserved qty

A new validation was recently added to prevent importing
reserved quantities on stock move lines, to prevent inventory
discrepancies [1]. However, such validation inadvertently introduced
another issue, which is now, if the reserved quantity is not filled with
0, the new validation is triggered.

This commit fixes the above issue by not requiring the reserved quantity
to be provided.

In addition, a typo is fixed in the error message:
"it is not allow" -> "allowed"

[1] odoo/odoo#119201

closes odoo/odoo#122871

X-original-commit: 76ad7b7d
Signed-off-by: Tiffany Chang <tic@odoo.com>