builder: use len() > 0 instead of != nil

Signed-off-by: Tibor Vass <teabee89@gmail.com>

builder/internals.go

@@ -532,7 +532,7 @@ func (b *Builder) create() (*daemon.Container, error) {
 	b.TmpContainers[c.ID] = struct{}{}
 	fmt.Fprintf(b.OutStream, " ---> Running in %s\n", utils.TruncateID(c.ID))
 
-	if config.Cmd != nil {
+	if len(config.Cmd) > 0 {
 		// override the entry point that may have been picked up from the base image
 		c.Path = config.Cmd[0]
 		c.Args = config.Cmd[1:]