chore: add comments about zero quality and test for it

Author: nd0ut

src/__tests__/convert-to-uploadcare-quality-string.spec.ts

@@ -16,4 +16,9 @@ describe('convertToUploadcareQualityString', () => {
   it('should map 100 to best', () => {
     expect(convertToUploadcareQualityString(100)).toBe('best');
   });
+
+  it('should falsy values to smart', () => {
+    expect(convertToUploadcareQualityString()).toBe('smart');
+    expect(convertToUploadcareQualityString(0)).toBe('smart');
+  });
 });

src/utils/helpers.ts

@@ -68,6 +68,8 @@ export function convertToUploadcareQualityString(
   requestedQuality?: number
 ): string {
   // If any particular quality has not been requested, we use the smart quality mode.
+  // zero quality is treated as no quality, so we use smart.
+  // see https://nextjs.org/docs/api-reference/next/image#quality
   if (!requestedQuality) {
     return 'smart';
   }