fix export diagram with wml variable (#615)

* Avoid exception at export when an xml process variable has not been
declared

closes [BS-16853](https://bonitasoft.atlassian.net/browse/BS-16853)

bundles/plugins/org.bonitasoft.studio.xml/src/org/bonitasoft/studio/xml/export/XSDResourceProvider.java

 /**
  * Copyright (C) 2010 BonitaSoft S.A.
  * BonitaSoft, 31 rue Gustave Eiffel - 38000 Grenoble
- *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
  * the Free Software Foundation, either version 2.0 of the License, or
  * (at your option) any later version.
- *
  * This program is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  * GNU General Public License for more details.
- *
  * You should have received a copy of the GNU General Public License
- * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
  */
 package org.bonitasoft.studio.xml.export;
 
 import java.util.HashSet;
 import java.util.List;
+import java.util.Optional;
 import java.util.Set;
 
 import org.bonitasoft.studio.common.emf.tools.ModelHelper;
@@ -35,24 +33,22 @@ import org.eclipse.emf.ecore.EObject;
 
 /**
  * @author Romain Bioteau
- *
  */
 public class XSDResourceProvider implements IBOSArchiveFileStoreProvider {
 
-
     @Override
     public Set<IRepositoryFileStore> getFileStoreForConfiguration(AbstractProcess process, Configuration configuration) {
-        final Set<IRepositoryFileStore> files = new HashSet<IRepositoryFileStore>() ;
-        final XSDRepositoryStore store = (XSDRepositoryStore) RepositoryManager.getInstance().getRepositoryStore(XSDRepositoryStore.class) ;
+        final Set<IRepositoryFileStore> files = new HashSet<IRepositoryFileStore>();
+        final XSDRepositoryStore store = (XSDRepositoryStore) RepositoryManager.getInstance()
+                .getRepositoryStore(XSDRepositoryStore.class);
 
         List<EObject> xmlData = ModelHelper.getAllItemsOfType(process, ProcessPackage.Literals.XML_DATA);
         for (EObject item : xmlData) {
-            XMLData data = (XMLData)item;
-            XSDFileStore artifact = store.findArtifactWithNamespace(data.getNamespace());
-            files.add(artifact) ;
+            XMLData data = (XMLData) item;
+            Optional<XSDFileStore> artifact = Optional.ofNullable(store.findArtifactWithNamespace(data.getNamespace()));
+            artifact.ifPresent(files::add);
         }
 
-
         return files;
     }