avoid empty "node []" and "edge []"

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

@@ -62,13 +62,17 @@ public class DotGraphWriter {
 	public void addDefaultNodeAttributes(final Map<String, String> attributes) throws IOException {
 		checkState(DotWriterState.STARTED);
 
-		writer.writeln(DotGraph.NODE + ' ' + assembleAttributes(attributes));
+		if (attributes != null && !attributes.isEmpty()) {
+			writer.writeln(DotGraph.NODE + ' ' + assembleAttributes(attributes));
+		}
 	}
 
 	public void addDefaultEdgeAttributes(final Map<String, String> attributes) throws IOException {
 		checkState(DotWriterState.STARTED);
 
-		writer.writeln(DotGraph.EDGE + ' ' + assembleAttributes(attributes));
+		if (attributes != null && !attributes.isEmpty()) {
+			writer.writeln(DotGraph.EDGE + ' ' + assembleAttributes(attributes));
+		}
 	}
 
 	public void addGraphAttribute(final String key, final String value) throws IOException {