fix(sign): remove password validation check in handleJoinSigning

Password is optional - remove the validation that required password
to be non-empty before joining a sign session.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

backend/mpc-system/services/service-party-app/src/pages/Sign.tsx

@@ -115,10 +115,7 @@ export default function Sign() {
       setError('请选择要使用的钱包份额');
       return;
     }
-    if (!password) {
-      setError('请输入解锁密码');
-      return;
-    }
+    // 密码是可选的，如果创建时没有设置密码，这里也不需要输入
 
     setIsLoading(true);
     setError(null);