[FIX] spreadsheet: compute aggregates resulting to zero

When the aggregated value is 0 (9000 + -9000 = 0), it displays an empty cell
in spreadsheet, instead of zero.

closes odoo/odoo#159148

Task: 3827502
Signed-off-by: Pierre Rousseau (pro) <pro@odoo.com>

addons/spreadsheet/static/src/pivot/pivot_model.js

@@ -289,7 +289,7 @@ export class SpreadsheetPivotModel extends PivotModel {
         const { cols, rows } = this._getColsRowsValuesFromDomain(domain);
         const group = JSON.stringify([rows, cols]);
         const values = this.data.measurements[group];
-        return (values && values[0][measure]) || "";
+        return (values && values[0][measure]) ?? "";
     }
 
     /**

addons/spreadsheet/static/tests/pivots/model/pivot_plugin_test.js

@@ -599,6 +599,29 @@ QUnit.module("spreadsheet > pivot plugin", {}, () => {
         assert.equal(getCellValue(model, "E4"), 95);
     });
 
+    QUnit.test("aggregate to 0", async function (assert) {
+        const serverData = getBasicServerData();
+        serverData.models.partner.records = [
+            { id: 1, name: "A", probability: 10 },
+            { id: 2, name: "B", probability: -10 },
+        ];
+
+        const { model } = await createSpreadsheetWithPivot({
+            serverData,
+            arch: /*xml*/ `
+                <pivot>
+                    <field name="name" type="row"/>
+                    <field name="probability" type="measure"/>
+                </pivot>`,
+        });
+        setCellContent(model, "A1", '=ODOO.PIVOT(1, "probability", "name", "A")');
+        setCellContent(model, "A2", '=ODOO.PIVOT(1, "probability", "name", "B")');
+        setCellContent(model, "A3", '=ODOO.PIVOT(1, "probability")');
+        assert.strictEqual(getEvaluatedCell(model, "A1").value, 10);
+        assert.strictEqual(getEvaluatedCell(model, "A2").value, -10);
+        assert.strictEqual(getEvaluatedCell(model, "A3").value, 0);
+    });
+
     QUnit.test("can import/export sorted pivot", async (assert) => {
         const spreadsheetData = {
             pivots: {