avoid empty attributes

src/main/java/kieker/analysis/graph/util/dot/DotGraphWriter.java

@@ -84,7 +84,7 @@ public class DotGraphWriter {
 	public void addNode(final String id, final Map<String, String> attributes) throws IOException {
 		checkState(DotWriterState.STARTED);
 
-		if (attributes == null) {
+		if (attributes == null || attributes.isEmpty()) {
 			writer.writeln('"' + id + '"');
 		} else {
 			writer.writeln('"' + id + '"' + ' ' + assembleAttributes(attributes));
@@ -105,7 +105,7 @@ public class DotGraphWriter {
 			edgeConnector = DotGraph.DIRECTED_EDGE_CONNECTOR;
 		}
 
-		if (attributes == null) {
+		if (attributes == null || attributes.isEmpty()) {
 			writer.writeln('"' + sourceId + '"' + ' ' + edgeConnector + ' ' + '"' + targetId + '"');
 		} else {
 			writer.writeln('"' + sourceId + '"' + ' ' + edgeConnector + ' ' + '"' + targetId + '"' + ' ' + assembleAttributes(attributes));