moved checking empty dsn to option implementation

Author: asmyasnikov

connection.go

@@ -253,9 +253,6 @@ func open(ctx context.Context, opts ...Option) (_ Connection, err error) {
 		}
 	}
 	for _, opt := range opts {
-		if opt == nil {
-			continue
-		}
 		err = opt(ctx, c)
 		if err != nil {
 			return nil, xerrors.WithStackTrace(err)

options.go

@@ -62,10 +62,10 @@ func WithRequestsType(requestsType string) Option {
 // Warning: WithConnectionString will be removed at next major release
 // (connection string will be required string param of ydb.Open)
 func WithConnectionString(connectionString string) Option {
-	if connectionString == "" {
-		return nil
-	}
 	return func(ctx context.Context, c *connection) error {
+		if connectionString == "" {
+			return nil
+		}
 		options, err := dsn.Parse(connectionString)
 		if err != nil {
 			return xerrors.WithStackTrace(