generate: Don't serialize Spec.Linux as an empty JSON object

The "does the marshaled JSON look like '{}'?" check is a pretty cheap
trick, but it was the easiest way I could think of for "is there
anything useful in here?".

Signed-off-by: W. Trevor King <wking@tremily.us>

generate/generate.go

@@ -188,6 +188,16 @@ func (g *Generator) Spec() *rspec.Spec {
 
 // Save writes the spec into w.
 func (g *Generator) Save(w io.Writer) error {
+	if g.spec.Linux != nil {
+		buf, err := json.Marshal(g.spec.Linux)
+		if err != nil {
+			return err
+		}
+		if string(buf) == "{}" {
+			g.spec.Linux = nil
+		}
+	}
+
 	data, err := json.MarshalIndent(g.spec, "", "\t")
 	if err != nil {
 		return err