_Msh_printSysTrap(): bugfix: don't print nonexistent modernish traps
_Msh_printSysTrap(): bugfix: don't print nonexistent modernish traps

bin/modernish:
- When printing the traps for a signal, if there are no modernish
  stack or emulated POSIX trap actions defined, but the builtin
  trap is still set to _Msh_doTraps <SIG> <VAR>, then we printed an
  empty trap, on the assumption that the signal is being ignored.
  That assumption has not been valid since commit 5e8b1ae1 (Sep
  2016). Since then, _Msh_doTraps() unsets the builtin trap and
  reissues the signal, so if there are no modernish trap actions,
  it's as if the trap is not set. In that case, print nothing.

modernish/modernish