Fix error on MySQL backend when bsToken null

This commit fixes an error when the bootstrap token for a device is NULL. Apple
specifies[0] the behaviour of `GetBootstrapToken` to return an empty body when
there is no bootstrap token for the device. This behaviour has been conformed
to.

[0]: https://developer.apple.com/documentation/devicemanagement/get_bootstrap_token

service/request.go

@@ -59,15 +59,19 @@ func CheckinRequest(svc Checkin, r *mdm.Request, bodyBytes []byte) ([]byte, erro
 			err = fmt.Errorf("setbootstraptoken service: %w", err)
 		}
 	case *mdm.GetBootstrapToken:
+		// https://developer.apple.com/documentation/devicemanagement/get_bootstrap_token
 		var bsToken *mdm.BootstrapToken
 		bsToken, err = svc.GetBootstrapToken(r, m)
 		if err != nil {
 			err = fmt.Errorf("getbootstraptoken service: %w", err)
 			break
 		}
-		respBytes, err = plist.Marshal(bsToken)
-		if err != nil {
-			err = fmt.Errorf("marshal bootstrap token: %w", err)
+		// If there is no bsToken, return an empty body
+		if bsToken != nil {
+			respBytes, err = plist.Marshal(bsToken)
+			if err != nil {
+				err = fmt.Errorf("marshal bootstrap token: %w", err)
+			}
 		}
 	case *mdm.DeclarativeManagement:
 		respBytes, err = svc.DeclarativeManagement(r, m)

storage/mysql/bstoken.go

@@ -1,6 +1,8 @@
 package mysql
 
 import (
+	"database/sql"
+
 	"github.com/micromdm/nanomdm/mdm"
 )
 
@@ -18,17 +20,17 @@ func (s *MySQLStorage) StoreBootstrapToken(r *mdm.Request, msg *mdm.SetBootstrap
 }
 
 func (s *MySQLStorage) RetrieveBootstrapToken(r *mdm.Request, _ *mdm.GetBootstrapToken) (*mdm.BootstrapToken, error) {
-	var tokenB64 string
+	var tokenB64 sql.NullString
 	err := s.db.QueryRowContext(
 		r.Context,
 		`SELECT bootstrap_token_b64 FROM devices WHERE id = ?;`,
 		r.ID,
 	).Scan(&tokenB64)
-	if err != nil {
+	if err != nil || !tokenB64.Valid {
 		return nil, err
 	}
 	bsToken := new(mdm.BootstrapToken)
-	err = bsToken.SetTokenString(tokenB64)
+	err = bsToken.SetTokenString(tokenB64.String)
 	if err == nil {
 		err = s.updateLastSeen(r)
 	}