add nodes and edges without attributes

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

@@ -77,10 +77,22 @@ public class DotWriter {
 		writer.writeln(assembleAttribute(key, value));
 	}
 
+	public void addNode(final String id) throws IOException {
+		addNode(id, null);
+	}
+
 	public void addNode(final String id, final Map<String, String> attributes) throws IOException {
 		checkState(DotWriterState.STARTED);
 
-		writer.writeln('"' + id + '"' + ' ' + assembleAttributes(attributes));
+		if (attributes == null) {
+			writer.writeln('"' + id + '"');
+		} else {
+			writer.writeln('"' + id + '"' + ' ' + assembleAttributes(attributes));
+		}
+	}
+
+	public void addEdge(final String sourceId, final String targetId) throws IOException {
+		addEdge(sourceId, targetId, null);
 	}
 
 	public void addEdge(final String sourceId, final String targetId, final Map<String, String> attributes) throws IOException {
@@ -92,7 +104,12 @@ public class DotWriter {
 		} else {
 			edgeConnector = DotGraph.DIRECTED_EDGE_CONNECTOR;
 		}
-		writer.writeln('"' + sourceId + '"' + ' ' + edgeConnector + ' ' + '"' + targetId + '"' + ' ' + assembleAttributes(attributes));
+
+		if (attributes == null) {
+			writer.writeln('"' + sourceId + '"' + ' ' + edgeConnector + ' ' + '"' + targetId + '"');
+		} else {
+			writer.writeln('"' + sourceId + '"' + ' ' + edgeConnector + ' ' + '"' + targetId + '"' + ' ' + assembleAttributes(attributes));
+		}
 	}
 
 	public void addSubgraphStart(final String name) throws IOException {