[FIX] web: JS traceback when trying to load unexisting field

Correction of PR:
https://github.com/odoo/odoo/pull/106101

According to experimental test and documentation:
https://developer.mozilla.org/en-US/docs/Web/API/Element/getAttribute
> If the given attribute does not exist, the value returned
will either be `null` or `""`
> All modern web browsers return `null` when the specified
attribute does not exist on the specified element.

OPW-3071843

closes odoo/odoo#114494

X-original-commit: a91717ab65e3bee5f4c8d4bb85c18458fd3263c8
Signed-off-by: Loan (LSE) <lse@odoo.com>
Signed-off-by: Aaron Bohy (aab) <aab@odoo.com>
Signed-off-by: Sens Loan (lse) <lse@odoo.com>

addons/web/static/src/legacy/legacy_load_views.js

@@ -65,7 +65,7 @@ export function processArch(arch, viewType, resModel, models) {
             if (viewFields[fieldName] === undefined) {
                 const viewFieldsInfo = models[resModel][fieldName];
                 const nodeString = node.getAttribute("string");
-                if (nodeString === undefined && viewFieldsInfo === undefined) {
+                if (nodeString === null && viewFieldsInfo === undefined) {
                     throw new Error(`Missing field string information for the field '${fieldName}' from the '${resModel}' model`);
                 }
                 viewFields[fieldName] = {