Fix basic auth failing when password field is empty or unset

Handle undefined username/password values by defaulting to empty string,
preventing "undefined" from being encoded in the Authorization header.

Fixes https://feedback.yaak.app/p/strange-basic-auth-behaviour-in-202612

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

plugins/auth-basic/package.json

@@ -11,6 +11,7 @@
   "version": "0.1.0",
   "scripts": {
     "build": "yaakcli build",
-    "dev": "yaakcli dev"
+    "dev": "yaakcli dev",
+    "test": "vitest --run tests"
   }
 }