perf[protocol]: 优化空字符串

protocol/src/main/java/com/zfoo/protocol/generate/GenerateProtocolPath.java

@@ -55,8 +55,8 @@ public abstract class GenerateProtocolPath {
 
     public static String getRelativePath(short protocolId, short relativeProtocolId) {
         // 不是折叠协议的话，protocolPathMap一定是空，这里返回“”，上层会解析为同一个文件下
-        if(protocolPathMap.size() == 0){
-            return "";
+        if(CollectionUtils.isEmpty(protocolPathMap)){
+            return StringUtils.EMPTY;
         }
         var protocolPath = protocolPathMap.get(protocolId);
         var relativePath = protocolPathMap.get(relativeProtocolId);