fix: valve NPE

Author: ReaJason

generator/src/main/java/com/reajason/javaweb/memshell/config/ShellConfig.java

@@ -26,7 +26,8 @@ public class ShellConfig {
     /**
      * 目标服务版本
      */
-    private String serverVersion;
+    @Builder.Default
+    private String serverVersion = "unknown";
 
     /**
      * 内存马功能