Planning fails if DeploymentArtifacts XML Tag is empty

[milesstoetzner]

If there is an empty DeploymentArtifacts XML Tag inside a Node Type Implementation then planning throws a Null Pointer Exception.
This happens e.g. when a Deployment Artifact is added in Winery and then removed.

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<Definitions targetNamespace="http://opentosca.org/nodetypeimplementations" id="nodeTypeImplementations-DockerContainer-Implementation_w1" xmlns="http://docs.oasis-open.org/tosca/ns/2011/12" xmlns:yml="http://docs.oasis-open.org/tosca/ns/simple/yaml/1.3" xmlns:selfservice="http://www.eclipse.org/winery/model/selfservice" xmlns:winery="http://www.opentosca.org/winery/extensions/tosca/2013/02/12" xmlns:testwineryopentoscaorg="http://test.winery.opentosca.org">
    <NodeTypeImplementation targetNamespace="http://opentosca.org/nodetypeimplementations" name="DockerContainer-Implementation_w1" abstract="no" final="no" nodeType="nodeTypes:DockerContainer_w1" xmlns:nodeTypes="http://opentosca.org/nodetypes">
        <ImplementationArtifacts>
            <ImplementationArtifact interfaceName="ContainerManagementInterface" name="DockerContainer_ContainerManagementInterface_IA" artifactType="artifactTypes:WAR-Java8" artifactRef="artifactTemplates:DockerContainer_ContainerManagementInterface-w1" xmlns:artifactTemplates="http://opentosca.org/artifacttemplates" xmlns:artifactTypes="http://opentosca.org/artifacttypes"/>
        </ImplementationArtifacts>
        <DeploymentArtifacts/>   <---- THIS WILL BREAK
    </NodeTypeImplementation>
</Definitions>