Implement regression test for empty logs loaded via nix log

tests/logging.sh

@@ -13,3 +13,14 @@ rm -rf $NIX_LOG_DIR
 (! nix-store -l $path)
 nix-build dependencies.nix --no-out-link --compress-build-log
 [ "$(nix-store -l $path)" = FOO ]
+
+# test whether empty logs work fine with `nix log`.
+builder="$(mktemp)"
+echo -e "#!/bin/sh\nmkdir \$out" > "$builder"
+outp="$(nix-build -E \
+    'with import ./config.nix; mkDerivation { name = "fnord"; builder = '"$builder"'; }' \
+    --out-link "$(mktemp -d)/result")"
+
+test -d "$outp"
+
+nix log "$outp"