fix: the issue that the useSlugify hook may cause the value to be und…

…efined (#884)

#### What type of PR is this?

/kind bug

#### What this PR does / why we need it:

修复新建分类和标签之后再次打开表单，别名显示为 undefined 的问题。

#### Which issue(s) this PR fixes:

Fixes halo-dev/halo#3382

#### Special notes for your reviewer:

测试方式：

1. 新建分类或者标签，创建完成之后再次打开创建表单，检查别名是否还为 undefined。

#### Does this PR introduce a user-facing change?

```release-note
修复 Console 端新建分类和标签之后再次打开表单，别名显示为 undefined 的问题。
```

src/composables/use-slugify.ts

@@ -15,6 +15,9 @@ export default function useSlugify(
   );
 
   const handleGenerateSlug = () => {
+    if (!source.value) {
+      return;
+    }
     target.value = slugify(source.value, {
       trim: true,
     });