libmain: don't print empty lines

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 #424

Change-Id: Iee793c79ba5a485d6606e0d292ed2eae6dfb7216

src/libmain/progress-bar.cc

@@ -396,7 +396,7 @@ std::chrono::milliseconds ProgressBar::draw(State & state, const std::optional<s
     if (printMultiline && moreActivities)
         writeToStderr(fmt("And %d more...", moreActivities));
 
-    if (!printMultiline) {
+    if (!printMultiline && !line.empty()) {
         line += " " + activity_line;
          writeToStderr("\r" + filterANSIEscapes(line, false, width) + ANSI_NORMAL + "\e[K");
     }