[FIX] mrp: fix forecast for product template

Following 873d6d92, the forecast would now redirect correctly to the
corresponding bom when a `product.product` was used.
The issue is that `bom_variant_ids` doesn't exist on `product.template`,
raising a traceback when the forecast was accessed through the product
themselves instead of their variants

closes odoo/odoo#115716

Signed-off-by: Arnold Moyaux (arm) <arm@odoo.com>

addons/mrp/static/src/mrp_forecasted/forecasted_buttons.js

@@ -8,8 +8,9 @@ patch(ForecastedButtons.prototype, 'mrp.ForecastedButtons',{
     setup() {
         this._super.apply();
         onWillStart(async () =>{
-            const res = (await this.orm.call(this.resModel, 'read', [this.productId], {fields: ['bom_ids', 'variant_bom_ids']}))[0];
-            this.bomId = res.variant_bom_ids[0] || res.bom_ids[0];
+            const fields = this.resModel === "product.template" ? ['bom_ids'] : ['bom_ids', 'variant_bom_ids'];
+            const res = (await this.orm.call(this.resModel, 'read', [this.productId], { fields }))[0];
+            this.bomId = res.variant_bom_ids ? res.variant_bom_ids[0] || res.bom_ids[0] : res.bom_ids[0];
         });
     },