fix(dashboard): 修正navbar组件prop传递方式

- 将leftIcon和leftText的undefined改为空字符串""
- 明确传递空值而非未定义，语义更清晰
- 虽然两种方式在渲染效果上相同，但空字符串更符合组件设计意图

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Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

mini-ui-packages/yongren-dashboard-ui/src/pages/Dashboard/Dashboard.tsx

@@ -138,8 +138,8 @@ const Dashboard: React.FC = () => {
         {/* 导航栏 */}
         <Navbar
           title="企业仪表板"
-          leftIcon={undefined}
-          leftText={undefined}
+          leftIcon=""
+          leftText=""
           onClickLeft={() => {}}
           backgroundColor="bg-white"
           border={true}