[release-2.0] fix: cannot save post when the publishTime is blank (#764)

This is an automated cherry-pick of #763

/assign JohnNiang

```release-note
修复 Console 端文章设置中的发布时间为空字符串时，无法保存的问题。
```

src/modules/contents/pages/components/SinglePageSettingModal.vue

@@ -177,7 +177,7 @@ const publishTime = computed(() => {
 });
 
 const onPublishTimeChange = (value: string) => {
-  formState.value.spec.publishTime = toISOString(value);
+  formState.value.spec.publishTime = value ? toISOString(value) : undefined;
 };
 </script>
 

src/modules/contents/posts/components/PostSettingModal.vue

@@ -169,7 +169,7 @@ const publishTime = computed(() => {
 });
 
 const onPublishTimeChange = (value: string) => {
-  formState.value.spec.publishTime = toISOString(value);
+  formState.value.spec.publishTime = value ? toISOString(value) : undefined;
 };
 </script>
 <template>