fix: apply the encode callback to empty cookie values

src/index.ts

@@ -292,7 +292,7 @@ export function stringifySetCookie(
     throw new TypeError(`argument name is invalid: ${cookie.name}`);
   }
 
-  const value = cookie.value ? enc(cookie.value) : "";
+  const value = cookie.value == null ? "" : enc(cookie.value);
 
   if (!cookieValueRegExp.test(value)) {
     throw new TypeError(`argument val is invalid: ${cookie.value}`);

src/stringify-set-cookie.spec.ts

@@ -126,6 +126,15 @@ describe("cookie.stringifySetCookie", () => {
       ).toEqual("foo=YmFy");
     });
 
+    it("should apply the encoder to an empty value", function () {
+      expect(
+        stringifySetCookie(
+          { name: "foo", value: "" },
+          { encode: (v) => "[" + v + "]" },
+        ),
+      ).toEqual("foo=[]");
+    });
+
     it.each(["foo=bar", 'foo"bar', "foo,bar", "foo\\bar", "foo$bar"])(
       "should serialize value: %s",
       (value) => {