Addresses #352

For now I'm just handling the case where the dataType variable is not defined.
But if we run headless legacy on Model 12.gaml from model library, we define the displays as 2d explicitly, so shouldn't dataType be 2d display ? And if so, what is the expected dataType in case of 3d display ?

gama.headless/src/gama/headless/xml/XMLWriter.java

@@ -12,6 +12,7 @@
 
 import java.io.*;
 
+import gama.headless.common.DataType;
 import gama.headless.core.*;
 import gama.headless.job.ExperimentJob;
 import gama.headless.job.ListenedVariable;
@@ -66,7 +67,9 @@ public void close() {
 	public void writeResultStep(final long step, final ListenedVariable[] vars) {
 		StringBuilder sb = new StringBuilder().append("\t<Step id='").append(step).append("' >\n");
 		for ( int i = 0; i < vars.length; i++ ) {
-			sb.append("\t\t<Variable name='").append(vars[i].getName()).append("' type='").append(vars[i].getDataType().name()).append("'>").append(vars[i].getValue())
+			sb.append("\t\t<Variable name='").append(vars[i].getName())
+								.append("' type='").append((vars[i].getDataType() == null ? DataType.UNDEFINED : vars[i].getDataType()).name())
+								.append("'>").append(vars[i].getValue())
 				.append("</Variable>\n");
 		}
 		sb.append("\t</Step>\n");