refactor: theme list data conversion (#2592)

#### What type of PR is this?
/kind bug
/area core
/milestone 2.0

#### What this PR does / why we need it:
修复主题列表数据类型转换问题
#### Which issue(s) this PR fixes:

Fixes #

#### Special notes for your reviewer:
how to test it?
访问如下 API 不报错即可
```
curl http://localhost:8090/apis/api.console.halo.run/v1alpha1/themes?uninstalled=false
```

/cc @halo-dev/sig-halo 
#### Does this PR introduce a user-facing change?

```release-note
None
```

src/main/java/run/halo/app/core/extension/endpoint/ThemeEndpoint.java

@@ -310,6 +310,7 @@ public class ThemeEndpoint implements CustomEndpoint {
             return walkThemesFromPath(themesDir)
                 .filter(Files::isDirectory)
                 .map(themePath -> loadUnstructured(themePath, THEME_MANIFESTS))
+                .flatMap(Flux::fromIterable)
                 .map(unstructured -> Unstructured.OBJECT_MAPPER.convertValue(unstructured,
                     Theme.class))
                 .sort(Comparator.comparing(theme -> theme.getMetadata().getName()));