Ensure specifying an empty platform is treated as default

The default platform had previously been provided using the
empty string. This change ensures that the platforms
field is always filled in correctly and empty string is
properly interpreted.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

client_opts.go

@@ -18,6 +18,7 @@ package containerd
 
 import (
 	"github.com/containerd/containerd/images"
+	"github.com/containerd/containerd/platforms"
 	"github.com/containerd/containerd/remotes"
 	"google.golang.org/grpc"
 )
@@ -76,6 +77,9 @@ type RemoteOpt func(*Client, *RemoteContext) error
 // WithPlatform allows the caller to specify a platform to retrieve
 // content for
 func WithPlatform(platform string) RemoteOpt {
+	if platform == "" {
+		platform = platforms.Default()
+	}
 	return func(_ *Client, c *RemoteContext) error {
 		for _, p := range c.Platforms {
 			if p == platform {