libmain: fix ignoring empty lines in the print-build-logs option

[momeemt]

Closes: #11991

Motivation

The logger previously ignored empty lines in the print-build-logs option.

Running the following nix file with this option shows that empty lines are not printed in the log, even though the build script prints them.

# repro.nix

let buildscript = builtins.toFile "build.sh" ''
	#!/bin/sh
	echo "start"
	echo
	echo
	echo
	echo "end"
	: > $out
'';
in
builtins.derivation {
	name = "repro";
	builder = "/bin/sh";
	args = [ buildscript ];
	outputs = [ "out" ];
	system = builtins.currentSystem;
}

$ nix build -f ./repro.nix -L --rebuild
repro> start
repro> end
$ nix log -f ./repro.nix | cat
start



end

I fixed the problem by removing !lastLine.empty(), so it now outputs like this.

$ nix build -f ./repro.nix -L --rebuild
repro> start
repro>
repro>
repro>
repro> end
$nix log -f ./repro.nix | cat
start



end

Context

This problem was reported by #11991.

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[Mic92]

https://discourse.nixos.org/t/2024-12-04-nix-team-meeting-minutes-200/57005

In our nix meeting we decided that it should not print repeated empty lines. A single empty line is fine.

[momeemt]

discourse.nixos.org/t/2024-12-04-nix-team-meeting-minutes-200/57005

In our nix meeting we decided that it should not print repeated empty lines. A single empty line is fine.

Please let me check just to be sure.
The current implementation does not output a blank line, is it correct to fix compressing to a single empty line?
Or is it to close this pull request?

[Mic92]

@momeemt So just now it emits no empty newline and we would be ok with emitting one empty newline and than discarding the rest. Unless we have a real good use case for multiple new lines, we think just having one empty newline makes logs more readable.