PBM-1387: return empty if not path is set (#1018)

Author: defbin

cmd/pbm/config.go

@@ -84,6 +84,9 @@ func runConfig(
 	case len(c.key) > 0:
 		k, err := config.GetConfigVar(ctx, conn, c.key)
 		if err != nil {
+			if errors.Is(err, config.ErrUnsetConfigPath) {
+				return confKV{c.key, ""}, nil // unset config path
+			}
 			return nil, errors.Wrap(err, "unable to get config key")
 		}
 		return confKV{c.key, fmt.Sprint(k)}, nil

pbm/config/config.go

@@ -15,6 +15,7 @@ import (
 	"go.mongodb.org/mongo-driver/bson/primitive"
 	"go.mongodb.org/mongo-driver/mongo"
 	"go.mongodb.org/mongo-driver/mongo/options"
+	"go.mongodb.org/mongo-driver/x/bsonx/bsoncore"
 	"gopkg.in/yaml.v2"
 
 	"github.com/percona/percona-backup-mongodb/pbm/compress"
@@ -32,6 +33,7 @@ var (
 	ErrUnkownStorageType   = errors.New("unknown storage type")
 	ErrMissedConfig        = errors.New("missed config")
 	ErrMissedConfigProfile = errors.New("missed config profile")
+	ErrUnsetConfigPath     = bsoncore.ErrElementNotFound
 )
 
 type confMap map[string]reflect.Kind