Handle falsy arguments passed to getters

In case `null` or `undefinded` is passed to one of the getter methods
we shouldn't treat it as if it was an attempt to get all cookies.

Fixes #399

src/js.cookie.mjs

@@ -123,9 +123,15 @@ function init (converter) {
 
   api.set = set
   api.get = function (key) {
-    return get(key, false /* read as raw */)
+    if (arguments.length && !key) {
+      return
+    }
+    return get(key /* read as raw */)
   }
   api.getJSON = function (key) {
+    if (arguments.length && !key) {
+      return
+    }
     return get(key, true /* read as json */)
   }
   api.remove = function (key, attributes) {

test/tests.js

@@ -106,6 +106,20 @@ QUnit.test('Call to read cookie when there is a window.json variable globally',
   delete window.json
 })
 
+QUnit.test('Passing `undefined` first argument', function (assert) {
+  assert.expect(2)
+  Cookies.set('foo', 'bar')
+  assert.strictEqual(Cookies.get(undefined), undefined, 'should not attempt to retrieve all cookies')
+  assert.strictEqual(Cookies.getJSON(undefined), undefined, 'should not attempt to retrieve all JSON cookies')
+})
+
+QUnit.test('Passing `null` first argument', function (assert) {
+  assert.expect(2)
+  Cookies.set('foo', 'bar')
+  assert.strictEqual(Cookies.get(null), undefined, 'should not attempt to retrieve all cookies')
+  assert.strictEqual(Cookies.getJSON(null), undefined, 'should not attempt to retrieve all JSON cookies')
+})
+
 QUnit.module('write', lifecycle)
 
 QUnit.test('String primitive', function (assert) {