Fix NPE when using jfx:native goal in 8.1.1

Hello,

In the first words I would like to thank you for this awesome Maven plugin :)

This pull request is to fix NPE when performing ```mvn clean jfx:native```:

My plugin setting:

```xml
<plugin>
	<groupId>com.zenjava</groupId>
	<artifactId>javafx-maven-plugin</artifactId>
	<version>8.1.1</version>
	<executions>
		<execution>
			<phase>package</phase>
			<goals>
				<goal>jar</goal>
			</goals>
		</execution>
	</executions>
	<configuration>
		<mainClass>WebCamAppLauncher</mainClass>
	</configuration>
</plugin>			
```

Exception I faced:

```plain
[ERROR] Failed to execute goal com.zenjava:javafx-maven-plugin:8.1.2-SNAPSHOT:native (default-cli) on project webcam-capture-example-javafx: An error occurred while generating native deployment bundles: NullPointerException -> [Help 1]
org.apache.maven.lifecycle.LifecycleExecutionException: Failed to execute goal com.zenjava:javafx-maven-plugin:8.1.2-SNAPSHOT:native (default-cli) on project webcam-capture-example-javafx: An error occurred while generating native deployment bundles
	at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:216)
	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:116)
	at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:80)
	at org.apache.maven.lifecycle.internal.builder.singlethreaded.SingleThreadedBuilder.build(SingleThreadedBuilder.java:51)
	at org.apache.maven.lifecycle.internal.LifecycleStarter.execute(LifecycleStarter.java:120)
	at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:347)
	at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:154)
	at org.apache.maven.cli.MavenCli.execute(MavenCli.java:584)
	at org.apache.maven.cli.MavenCli.doMain(MavenCli.java:213)
	at org.apache.maven.cli.MavenCli.main(MavenCli.java:157)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:483)
	at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced(Launcher.java:289)
	at org.codehaus.plexus.classworlds.launcher.Launcher.launch(Launcher.java:229)
	at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode(Launcher.java:415)
	at org.codehaus.plexus.classworlds.launcher.Launcher.main(Launcher.java:356)
Caused by: org.apache.maven.plugin.MojoExecutionException: An error occurred while generating native deployment bundles
	at com.zenjava.javafx.maven.plugin.NativeMojo.execute(NativeMojo.java:294)
	at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo(DefaultBuildPluginManager.java:132)
	at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:208)
	... 19 more
Caused by: java.lang.NullPointerException
	at com.zenjava.javafx.maven.plugin.NativeMojo.execute(NativeMojo.java:267)
	... 21 more
```

I tested this fix and was able to generate ```*.deb``` and ```*.rpm``` packages successfully.

Thanks!