feat(media): 添加空状态检查工具函数并优化单媒体编辑器逻辑

新增 shouldShowMediaEmptyState 工具函数用于判断是否显示媒体空状态，为其编写单元测试覆盖核心场景，同时在单媒体编辑器组件中使用该函数替换原有的内联条件判断，提升代码可维护性。

WonderQ-Admin-UI-Vue/src/components/media/SingleMediaEditor.vue

@@ -2,7 +2,7 @@
 import { ref } from "vue";
 import { uploadMediaAsset } from "@/api/client";
 import { validateMediaFile } from "@/lib/media";
-import { resolveAdminMediaUrl } from "@/lib/media-url";
+import { resolveAdminMediaUrl, shouldShowMediaEmptyState } from "@/lib/media-url";
 
 const props = defineProps<{
   modelValue: string;
@@ -58,7 +58,7 @@ function clear() {
     <el-alert v-if="error" :title="error" type="error" :closable="false" show-icon />
     <el-image v-if="modelValue" class="single-media-preview" :src="previewUrl(modelValue)" fit="cover" :preview-src-list="[previewUrl(modelValue) ?? modelValue]" @error="previewError = true" />
     <el-alert v-if="previewError" title="图片加载失败，请重新上传" type="warning" :closable="false" show-icon />
-    <el-empty v-else :image-size="48" description="暂无图片" />
+    <el-empty v-if="shouldShowMediaEmptyState(modelValue, previewError)" :image-size="48" description="暂无图片" />
     <el-button v-if="modelValue" link type="danger" :disabled="disabled || uploading" @click="clear">移除图片</el-button>
   </div>
 </template>

WonderQ-Admin-UI-Vue/src/lib/media-url.test.ts

@@ -1,5 +1,5 @@
 import { describe, expect, it } from "vitest";
-import { resolveAdminMediaUrl, resolveAdminMediaUrls } from "@/lib/media-url";
+import { resolveAdminMediaUrl, resolveAdminMediaUrls, shouldShowMediaEmptyState } from "@/lib/media-url";
 
 describe("resolveAdminMediaUrl", () => {
   it("将公共本地资源挂载到管理端 base 下", () => {
@@ -18,4 +18,9 @@ describe("resolveAdminMediaUrl", () => {
       "https://example.com/b.jpg",
     ]);
   });
+
+  it("已有图片地址时不显示暂无图片占位", () => {
+    expect(shouldShowMediaEmptyState("https://example.com/image.webp", false)).toBe(false);
+    expect(shouldShowMediaEmptyState("", false)).toBe(true);
+  });
 });

WonderQ-Admin-UI-Vue/src/lib/media-url.ts

@@ -17,3 +17,7 @@ export function resolveAdminMediaUrl(value: string | null | undefined, baseUrl =
 export function resolveAdminMediaUrls(values: string[], baseUrl = import.meta.env.BASE_URL): string[] {
   return values.map((value) => resolveAdminMediaUrl(value, baseUrl) ?? "");
 }
+
+export function shouldShowMediaEmptyState(value: string | null | undefined, previewFailed: boolean): boolean {
+  return !value && !previewFailed;
+}