Cannot generate <packaging> element in Maven POM

[snowdream]

https://github.com/SnowdreamFramework/annotation

@dcendents

i have configure as follows:

private configurePOM(def pom) {
    pom.project {
        groupId optProperty("POM_GROUP_ID")
        artifactId optProperty("POM_ARTIFACT_ID")
        version optProperty("POM_VERSION")
        name optProperty("POM_NAME")
        packaging optProperty("POM_PACKAGING")
        description optProperty("POM_DESCRIPTION")
        url optProperty("POM_URL")
        inceptionYear optProperty("POM_INCEPTION_YEAR")

        scm {
            url optProperty("POM_SCM_URL")
            connection optProperty("POM_SCM_CONNECTION")
            developerConnection optProperty("POM_SCM_DEV_CONNECTION")
        }

        licenses {
            license {
                name optProperty("POM_LICENCE_NAME")
                url optProperty("POM_LICENCE_URL")
                distribution optProperty("POM_LICENCE_DIST")
                comments optProperty("POM_LICENCE_COMMENTS")
            }
        }

        developers {
            developer {
                id optProperty("POM_DEVELOPER_ID")
                name optProperty("POM_DEVELOPER_NAME")
                email optProperty("POM_DEVELOPER_EMAIL")
                url optProperty("POM_DEVELOPER_URL")
            }
        }

        issueManagement {
            system optProperty("POM_ISSUE_MANAGEMENT_SYSTEM")
            url optProperty("POM_ISSUE_MANAGEMENT_URL")
        }
    }
}

but it seems < packaging > element is missing.

<?xml version="1.0" encoding="UTF-8"?>
<project xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd" xmlns="http://maven.apache.org/POM/4.0.0"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <modelVersion>4.0.0</modelVersion>
  <groupId>com.github.snowdream</groupId>
  <artifactId>annotation</artifactId>
  <version>0.2</version>
  <name>annotation</name>
  <description>SnowdreamFramework</description>
  <url>https://github.com/SnowdreamFramework/annotation</url>
  <inceptionYear>2017</inceptionYear>
  <licenses>
    <license>
      <name>The Apache Software License, Version 2.0</name>
      <url>http://www.apache.org/licenses/LICENSE-2.0.txt</url>
      <distribution>repo</distribution>
      <comments>A business-friendly OSS license</comments>
    </license>
  </licenses>
  <developers>
    <developer>
      <id>snowdream</id>
      <name>YangHui</name>
      <email>yanghui1986527@gmail.com</email>
      <url>http://snowdream.github.io</url>
    </developer>
  </developers>
  <scm>
    <connection>scm:git:git@github.com:SnowdreamFramework/annotation.git</connection>
    <developerConnection>scm:git:git@github.com:SnowdreamFramework/annotation.git</developerConnection>
    <url>git@github.com:SnowdreamFramework/annotation.git</url>
  </scm>
  <issueManagement>
    <system>Github</system>
    <url>https://github.com/SnowdreamFramework/annotation/issues</url>
  </issueManagement>
  <dependencies>
    <dependency>
      <groupId>org.jetbrains.kotlin</groupId>
      <artifactId>kotlin-stdlib</artifactId>
      <version>1.0.5-2</version>
      <scope>compile</scope>
    </dependency>
  </dependencies>
</project>