Fix buf graph for unattached nodes (#2244)

private/bufpkg/bufgraph/builder.go

@@ -97,6 +97,7 @@ func (b *builder) buildForModule(
 	workspace bufmodule.Workspace,
 	graph *dag.Graph[Node],
 ) ([]bufanalysis.FileAnnotation, error) {
+	graph.AddNode(node)
 	image, fileAnnotations, err := b.imageBuilder.Build(
 		ctx,
 		module,

private/pkg/dag/dag.go

@@ -244,7 +244,6 @@ func (g *Graph[Key]) DOTString(keyToString func(Key) string) (string, error) {
 	// We also want to pick up any nodes that do not have edges, and display them.
 	if err := g.WalkNodes(
 		func(key Key, inboundEdges []Key, outboundEdges []Key) error {
-			//
 			if _, ok := keyToIndex[key]; ok {
 				return nil
 			}