chore: set pb.Empty on ssh and secret mounts

These mounts have no vertex inputs, and so should be *explicitly*
marshaled as such.

Signed-off-by: Justin Chadwell <me@jedevc.com>
(cherry picked from commit 157efce)

client/llb/exec.go

@@ -396,6 +396,7 @@ func (e *ExecOp) Marshal(ctx context.Context, c *Constraints) (digest.Digest, []
 			})
 		} else {
 			pm := &pb.Mount{
+				Input:     pb.Empty,
 				Dest:      s.Target,
 				MountType: pb.MountType_SECRET,
 				SecretOpt: &pb.SecretOpt{
@@ -412,6 +413,7 @@ func (e *ExecOp) Marshal(ctx context.Context, c *Constraints) (digest.Digest, []
 
 	for _, s := range e.ssh {
 		pm := &pb.Mount{
+			Input:     pb.Empty,
 			Dest:      s.Target,
 			MountType: pb.MountType_SSH,
 			SSHOpt: &pb.SSHOpt{