fix nullPointerException

src/main/java/com/zzsn/knowbase/controller/KnowledgeController.java

@@ -75,7 +75,7 @@ public class KnowledgeController {
     @GetMapping(value = "/getById")
     public Result<?> getById(@RequestParam String id)  {
         KnowledgeVO knowledgeVO=knowledgeService.getById(id);
-        if(!knowledgeVO.getPublishDate().isEmpty()){
+        if(null!=knowledgeVO.getPublishDate()&&(!knowledgeVO.getPublishDate().isEmpty())){
             knowledgeVO.setPublishDate(EsDateUtil.esFieldDateMapping(knowledgeVO.getPublishDate()));
         }
         if(null!=knowledgeVO.getVerifyTime()&&(!knowledgeVO.getVerifyTime().isEmpty())){