test: cases to querystring related to empty string

+ Add cases to `qs.stringify` that return empty string
+ Add cases to `qs.parse` when `sep` or `eq` is empty

PR-URL: #11329
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

test/parallel/test-querystring.js

@@ -231,8 +231,20 @@ assert.doesNotThrow(function() {
   assert.equal(f, 'a:b;q:x%3Ay%3By%3Az');
 }
 
+// empty string
+assert.strictEqual(qs.stringify(), '');
+assert.strictEqual(qs.stringify(0), '');
+assert.strictEqual(qs.stringify([]), '');
+assert.strictEqual(qs.stringify(null), '');
+assert.strictEqual(qs.stringify(true), '');
+
 check(qs.parse(), {});
 
+// empty sep
+check(qs.parse('a', []), { a: '' });
+
+// empty eq
+check(qs.parse('a', null, []), { '': 'a' });
 
 // Test limiting
 assert.equal(