Fix <none>-tag nullpointer bug

artifacttemplates/http%3A%2F%2Fopentosca.org%2Fartifacttemplates/DockerEngine-IA-Manage/source/src/main/java/org/opentosca/nodetypes/org_opentosca_nodetypes_DockerEngine_IA_Manage.java

@@ -565,9 +565,9 @@ public class org_opentosca_nodetypes_DockerEngine_IA_Manage extends AbstractIASe
 	private String isImageAvailable(final String image, final DockerClient dockerClient) {
 		System.out.println("Searching available Images: ");
 		for (final Image availImage : dockerClient.listImagesCmd().exec()) {
-			if (availImage != null) {
+			if (availImage != null && availImage.getRepoTags() != null) {
 				for (final String tag : availImage.getRepoTags()) {
-					if (tag.startsWith(image)) {
+					if (tag != null && tag.startsWith(image)) {
 						return availImage.getId();
 					}
 				}