this most notably affects `nix eval`: if there is no progress bar to be
shown and no activities going on we should not print anything at all. a
progress bar with no activities would print a bunch of terminal escapes
*and a space*, which is not helpful in simple cases like nix eval -E 1.
notably this does *not* affect nix eval called on non-terminal outputs,
but it is slightly confusing nevertheless (and not difficult to avoid).
fixes lix-project/lix#424
Change-Id: Iee793c79ba5a485d6606e0d292ed2eae6dfb7216
eldritch horrors
3dd7d023f4