fix(backend): DietPreferencesController 处理 familyMemberId 为 null 时优雅降级

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

cfc-backend/src/main/java/com/etotem/cfc/controller/diet/DietPreferencesController.java

@@ -16,7 +16,10 @@ public class DietPreferencesController {
 
     @PostMapping("/current-member")
     public Result<DietPreferencesDTO> getCurrentMemberPreferences(
-            @RequestAttribute("familyMemberId") Long familyMemberId) {
+            @RequestAttribute(value = "familyMemberId", required = false) Long familyMemberId) {
+        if (familyMemberId == null) {
+            return Result.success(null);
+        }
         DietPreferencesDTO dto = dietPreferencesService.getCurrentMemberPreferences(familyMemberId);
         return Result.success(dto);
     }