fix: 可选字段处理

- 为搜索条目中的可选字段进行判断并处理

server/utils/search-converters.test.ts

@@ -12,6 +12,7 @@ describe('converters', () => {
       summary: 'High efficiency',
       description: 'Detailed description',
       type: 'pump',
+      cover: 'rand-om__-uuid-1234',
     };
     const result = converters.products(item);
     expect(result).toEqual({
@@ -19,6 +20,7 @@ describe('converters', () => {
       type: 'product',
       title: 'Hydraulic Pump',
       summary: 'High efficiency',
+      thumbnail: '/api/assets/rand-om__-uuid-1234',
     });
   });
 
@@ -29,6 +31,7 @@ describe('converters', () => {
       summary: 'Effective solution',
       content: 'Detailed content',
       type: 'Type A',
+      cover: 'rand-om__-uuid-5678',
     };
     const result = converters.solutions(item);
     expect(result).toEqual({
@@ -36,6 +39,7 @@ describe('converters', () => {
       type: 'solution',
       title: 'Solution A',
       summary: 'Effective solution',
+      thumbnail: '/api/assets/rand-om__-uuid-5678',
     });
   });