Error: The input doesn't contain any classes. Did you specify the proper '-injars' options?

执行Maven Install打包的时候，出现以下错误信息：

[proguard] ProGuard, version 4.4
 [proguard] Reading program war [F:\Workspaces\pro-test\target\pro-test-0.0.1-SNAPSHOT.war] (filtered)
 [proguard] Error: The input doesn‘t contain any classes. Did you specify the proper ‘-injars‘ options?

由于这个是一个测试工程，还没有添加任何的类，所以虽然POM文件中指定了“injars”参数，还是产生了此错误。

“injars”参数设置参考：

            <plugin>
                <groupId>com.pyx4me</groupId>
                <artifactId>proguard-maven-plugin</artifactId>
                <version>2.0.4</version>
                <executions>
                    <execution>
                        <phase>package</phase>
                        <goals>
                            <goal>proguard</goal>
                        </goals>
                    </execution>
                </executions>
                <configuration>
                    <obfuscate>true</obfuscate>
                    <proguardInclude>${basedir}/proguard.conf</proguardInclude>
                    <!-- 添加依赖，这里你可以按你的需要修改 -->
                    <libs>
                        <lib>${java.home}/lib/rt.jar</lib>
                        <lib>lib/jsp-api.jar</lib>
                        <lib>lib/servlet-api.jar</lib>
                    </libs>
                    <addMavenDescriptor>false</addMavenDescriptor>
                    <injar>${project.build.finalName}.war</injar>
                    <outjar>${project.build.finalName}-pg.war</outjar>
                    <outputDirectory>${project.build.directory}</outputDirectory>
                </configuration>
                <dependencies>
                    <!-- 使用4.4版本来混淆 -->
                    <dependency>
                        <groupId>net.sf.proguard</groupId>
                        <artifactId>proguard</artifactId>
                        <version>4.4</version>
                        <scope>runtime</scope>
                    </dependency>
                </dependencies>
            </plugin>

Error: The input doesn't contain any classes. Did you specify the proper '-injars' options?