fix(login): 修正租户选择逻辑及默认选项设置

- 在租户选项中添加“无”作为默认选项，value设为空字符串
- 修正提交时租户代码的判断，避免null或undefined导致错误提示
- 确保租户选择验证更加严格，提升用户体验

src/views/_builtin/login/modules/pwd-login.vue

@@ -24,10 +24,13 @@ const selectedTenantCode = ref<string | null>(null);
 async function loadTenantOptions() {
   const { data } = await fetchGetTenantOptions();
   if (data) {
-    tenantOptions.value = (data as any[]).map((item: any) => ({
-      label: item.tenantName,
-      value: item.tenantCode
-    }));
+    tenantOptions.value = [
+      { label: '无', value: '' },
+      ...(data as any[]).map((item: any) => ({
+        label: item.tenantName,
+        value: item.tenantCode
+      }))
+    ];
   }
 }
 loadTenantOptions();
@@ -68,7 +71,7 @@ const rules = computed<Record<keyof FormModel, App.Global.FormRule[]>>(() => {
 const captchaKey = ref('');
 async function handleSubmit() {
   await validate();
-  if (!selectedTenantCode.value) {
+  if (selectedTenantCode.value === null || selectedTenantCode.value === undefined) {
     window.$message?.error('请选择租户');
     return;
   }