mpt: move empty hash node in a separate type

We use them quite frequently (consider children for a new branch
node) and it is better to get rid of unneeded allocations.

Signed-off-by: Evgeniy Stratonikov <evgeniy@nspcc.ru>

pkg/core/mpt/empty.go

@@ -0,0 +1,53 @@
+package mpt
+
+import (
+	"encoding/json"
+	"errors"
+
+	"github.com/nspcc-dev/neo-go/pkg/io"
+	"github.com/nspcc-dev/neo-go/pkg/util"
+)
+
+// EmptyNode represents empty node.
+type EmptyNode struct{}
+
+// DecodeBinary implements io.Serializable interface.
+func (e EmptyNode) DecodeBinary(*io.BinReader) {
+}
+
+// EncodeBinary implements io.Serializable interface.
+func (e EmptyNode) EncodeBinary(*io.BinWriter) {
+}
+
+// MarshalJSON implements Node interface.
+func (e EmptyNode) MarshalJSON() ([]byte, error) {
+	return []byte(`{}`), nil
+}
+
+// UnmarshalJSON implements Node interface.
+func (e EmptyNode) UnmarshalJSON(bytes []byte) error {
+	var m map[string]interface{}
+	err := json.Unmarshal(bytes, &m)
+	if err != nil {
+		return err
+	}
+	if len(m) != 0 {
+		return errors.New("expected empty node")
+	}
+	return nil
+}
+
+// Hash implements Node interface.
+func (e EmptyNode) Hash() util.Uint256 {
+	panic("can't get hash of an EmptyNode")
+}
+
+// Type implements Node interface.
+func (e EmptyNode) Type() NodeType {
+	return EmptyT
+}
+
+// Bytes implements Node interface.
+func (e EmptyNode) Bytes() []byte {
+	return nil
+}