fix: theme page cannot be opened after the development environment is initialized (#833)

#### What type of PR is this?

/kind bug

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

修复在开发环境下首次初始化后，没有加载默认主题的情况下无法访问主题管理页面的问题。

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

Fixes https://github.com/halo-dev/halo/issues/3187

#### Special notes for your reviewer:

测试方式：

1. 在开发环境使用全新的数据库初始化。
2. 检查是否能够正常的访问主题管理页面。

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

```release-note
None
```

src/stores/theme.ts

@@ -19,7 +19,9 @@ export const useThemeStore = defineStore("theme", () => {
         mute: true,
       });
 
-      activatedTheme.value = data;
+      if (data) {
+        activatedTheme.value = data;
+      }
     } catch (e) {
       console.error("Failed to fetch active theme", e);
     }