:bug: 修复代码生成器生成的 Kotlin 代码会出现编译错误的问题，及修正内置的两个模板行为不一致的问题

Fix https://gitee.com/baomidou/mybatis-plus/issues/IPB3Y

mybatis-plus-extension/src/main/java/com/baomidou/mybatisplus/extension/toolkit/PackageHelper.java

@@ -67,7 +67,7 @@ public class PackageHelper {
                 }
             }
             if (!set.isEmpty()) {
-                return set.toArray(new String[]{});
+                return set.toArray(new String[] {});
             }
             return new String[0];
         } catch (Exception e) {

mybatis-plus-generator/src/main/resources/templates/entity.kt.ftl

@@ -82,7 +82,7 @@ class ${entity} : Serializable {
 
 </#if>
 <#if activeRecord>
-    override fun pkVal(): Serializable {
+    override fun pkVal(): Serializable? {
 <#if keyPropertyName??>
         return ${keyPropertyName}
 <#else>

mybatis-plus-generator/src/main/resources/templates/entity.kt.vm

@@ -77,11 +77,11 @@ class ${entity} : Serializable {
 
 #end
 #if(${activeRecord})
-    override fun pkVal(): Serializable {
+    override fun pkVal(): Serializable? {
 #if(${keyPropertyName})
-        return ${keyPropertyName}!!
+        return ${keyPropertyName}
 #else
-        return id!!
+        return null
 #end
     }
 

mybatis-plus-generator/src/test/java/com/baomidou/mybatisplus/test/generator/MysqlGenerator.java

@@ -69,7 +69,7 @@ public class MysqlGenerator extends GeneratorTest {
         ).setDataSource(
             // 数据源配置
             new DataSourceConfig()
-                .setDbType(DbType.MYSQL)// 数据库类型
+                .setDbType(DbType.MYSQL) // 数据库类型
                 .setTypeConvert(new MySqlTypeConvert() {
                     // 自定义数据库表字段类型转换【可选】
                     @Override