Cf. #3895 (comment)

Hi @MrBoriska ,
getting counter values requires context but this will totally work:

#context {
  let total_part = part_count.final().at(0)
  if total_part >= 0 [#total_part parts]
}

The difference is, that the counter's value is not converted to content (until the second line, in [#total_part parts]), achieved by wrapping all of it in one context block.