Don't 500 when generating a name for an empty result.

Refs #22310

src/main/java/info/textgrid/services/aggregator/AbstractExporter.java

@@ -403,9 +403,11 @@ private String calculateFilename() throws MetadataParseFault, ObjectNotFoundFaul
 		final StringBuilder result = new StringBuilder(title.length() + 30);
 		result.append(DefaultFilenamePolicy.INSTANCE.translate(title));
 		Iterator<ObjectType> roots = getRootObjects().iterator();
-		ObjectType first = roots.next();
-		if (!roots.hasNext())
-			result.append('.').append(URI.create(first.getGeneric().getGenerated().getTextgridUri().getValue()).getSchemeSpecificPart());
+		if (roots.hasNext()) {
+			ObjectType first = roots.next();
+			if (!roots.hasNext())
+				result.append('.').append(URI.create(first.getGeneric().getGenerated().getTextgridUri().getValue()).getSchemeSpecificPart());
+		}
 		if (fileExtension != null && !fileExtension.isEmpty())
 			result.append('.').append(fileExtension);
 		return result.toString();