Fixes a regression introduced in
48b444304e whereby "guix pack -C none
hello" would fail with:
ERROR: In procedure string-join:
In procedure string-join: Wrong type argument in position 1: #f
builder for `/gnu/store/…-tarball-pack.tar.drv' failed with exit code 1
* guix/scripts/pack.scm (self-contained-tarball): Adjust for when
'compressor-command' returns #f.