lib/repo: Properly handle NULL homedir when signing commit
lib/repo: Properly handle NULL homedir when signing commit

Without this, ostree_repo_sign_commit throws a critical message when no
homedir is provided:

(ostree gpg-sign:5034): GLib-GIO-CRITICAL **: g_file_new_for_path: assertion 'path != NULL' failed

Closes: #1305
Approved by: cgwalters

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree