fix conversion error with or on futures with {.async: (raises: []).}

[etan-status]

import chronos

proc f(): Future[void] {.async: (raises: []).} =
  discard

discard f() or f() or f()

/Users/etan/Documents/Repos/nimbus-eth2/vendor/nim-chronos/chronos/internal/raisesfutures.nim(145, 44) union
/Users/etan/Documents/Repos/nimbus-eth2/vendor/nimbus-build-system/vendor/Nim/lib/core/macros.nim(185, 28) []
/Users/etan/Documents/Repos/nimbus-eth2/test.nim(6, 13) template/generic instantiation of `or` from here
/Users/etan/Documents/Repos/nimbus-eth2/vendor/nim-chronos/chronos/internal/asyncfutures.nim(1668, 39) template/generic instantiation of `union` from here
/Users/etan/Documents/Repos/nimbus-eth2/vendor/nimbus-build-system/vendor/Nim/lib/core/macros.nim(185, 28) Error: illegal conversion from '-1' to '[0..9223372036854775807]'

Fix by checking for void before trying to access raises