fix(shopping): omit units for ingredients without quantities

tests/unit/calculator.test.ts

@@ -93,4 +93,12 @@ describe('Precise scaling from immutable originals', () => {
     );
     expect(b).toHaveLength(3);
   });
+  it('does not display an artificial unit for unquantified shopping ingredients', () => {
+    const list = mergeShopping(
+      [],
+      [{ ...chicken, name: 'Salz', amount: null, note: 'nach Geschmack' }],
+      '4',
+    );
+    expect(list[0]).toMatchObject({ amount: null, unit: '', note: 'nach Geschmack' });
+  });
 });