Don't break build if configured directory isn't available

[barthel]

To prevent something like:

[ERROR] Parameter 'directory' is not a directory
java.lang.IllegalArgumentException: Parameter 'directory' is not a directory
    at org.apache.commons.io.FileUtils.validateListFilesParameters(FileUtils.java:545)
    at org.apache.commons.io.FileUtils.listFiles(FileUtils.java:521)
    at org.apache.commons.io.FileUtils.iterateFiles(FileUtils.java:628)
    at de.icongmbh.oss.maven.plugin.javassist.ClassTransformer.iterateClassnames(ClassTransformer.java:198)
    at de.icongmbh.oss.maven.plugin.javassist.ClassTransformer.transform(ClassTransformer.java:131)
    at de.icongmbh.oss.maven.plugin.javassist.JavassistTransformerExecutor.execute(JavassistTransformerExecutor.java:107)
    at de.icongmbh.oss.maven.plugin.javassist.JavassistTransformerExecutor.execute(JavassistTransformerExecutor.java:98)
    at de.icongmbh.oss.maven.plugin.javassist.JavassistMojo.execute(JavassistMojo.java:118)
    at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo(DefaultBuildPluginManager.java:106)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:208)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:153)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:145)
    at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:84)
    at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:59)
    at org.apache.maven.lifecycle.internal.LifecycleStarter.singleThreadedBuild(LifecycleStarter.java:183)
    at org.apache.maven.lifecycle.internal.LifecycleStarter.execute(LifecycleStarter.java:161)
    at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:317)
    at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:152)
    at org.jvnet.hudson.maven3.launcher.Maven31Launcher.main(Maven31Launcher.java:132)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at org.codehaus.plexus.classworlds.launcher.Launcher.launchStandard(Launcher.java:330)
    at org.codehaus.plexus.classworlds.launcher.Launcher.launch(Launcher.java:238)
    at jenkins.maven3.agent.Maven31Main.launch(Maven31Main.java:181)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at hudson.maven.Maven3Builder.call(Maven3Builder.java:134)
    at hudson.maven.Maven3Builder.call(Maven3Builder.java:69)
    at hudson.remoting.UserRequest.perform(UserRequest.java:118)
    at hudson.remoting.UserRequest.perform(UserRequest.java:48)
    at hudson.remoting.Request$2.run(Request.java:328)
    at hudson.remoting.InterceptingExecutorService$1.call(InterceptingExecutorService.java:72)
    at java.util.concurrent.FutureTask.run(FutureTask.java:262)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
    at java.lang.Thread.run(Thread.java:745)
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------

For example in maven assembly projects where no target/classes (buildDir) exists and the plugin is defined in parent POM.