Fix bug where color palette could not be empty

__tests__/flattenColorPalette.test.js

@@ -75,3 +75,7 @@ test('it flattens deeply nested color objects', () => {
     'button-primary-focus-variant': 'orange',
   })
 })
+
+test('it handles empty objects', () => {
+  expect(flattenColorPalette({})).toEqual({})
+})

src/util/flattenColorPalette.js

@@ -1,5 +1,6 @@
 const flattenColorPalette = (colors) =>
   Object.assign(
+    {},
     ...Object.entries(colors).flatMap(([color, values]) =>
       typeof values == 'object'
         ? Object.entries(flattenColorPalette(values)).map(([number, hex]) => ({