优化标签打印出现null值，修复为留空

ruoyi-common/src/main/java/com/ruoyi/common/utils/MyUtils.java

@@ -28,7 +28,7 @@ public class MyUtils {
                 input = input.replace(placeholder, filedValue);
             } catch (Exception e) {
                 String placeholder = "${" + replacement + "}";
-                input = input.replace(placeholder, "null");
+                input = input.replace(placeholder, "");
             }
         }
         return input;