Treat an empty message as nil

In google.golang.org/protobuf, an empty message and nil are interchangeable.

golang/protobuf#965

Signed-off-by: Kazuyoshi Kato <[email protected]>

Author: kzys

client/llb/definition.go

@@ -29,9 +29,11 @@ type DefinitionOp struct {
 	inputCache *sync.Map // shared and written among DefinitionOps so avoid race on this map using sync.Map
 }
 
+var empty = &pb.Definition{}
+
 // NewDefinitionOp returns a new operation from a marshalled definition.
 func NewDefinitionOp(def *pb.Definition) (*DefinitionOp, error) {
-	if def == nil {
+	if def == nil || proto.Equal(def, empty) {
 		return nil, errors.New("invalid nil input definition to definition op")
 	}