In some cases the $sourceRoot is missing. Skip the hook instead
of showing the following cryptic error:
find: cannot search `': No such file or directory
/nix/store/0p1afvl8jcpi6dvsq2n58i90w9c59vz1-set-source-date-epoch-to-latest.sh: line 12: [: : integer expression expected
vcunat removed the warning; the hook will just skip silently in these cases.
Perhaps someone can improve on it some time.
