test: add test case for bug

Author: kenjis

tests/Unit/UserModelTest.php

@@ -45,6 +45,20 @@ public function testSaveInsertUser(): void
         ]);
     }
 
+    /**
+     * @see https://github.com/codeigniter4/shield/issues/546
+     */
+    public function testfindByCredentialsEmailEmptyString(): void
+    {
+        $users = $this->createUserModel();
+        $user  = $this->createNewUser();
+        $users->save($user);
+
+        $user = $users->findByCredentials(['email' => '']);
+
+        $this->assertNull($user);
+    }
+
     public function testInsertUserObject(): void
     {
         $users = $this->createUserModel();