ui-blob: don't segfault when no path is given

It it possible to inspect blobs by specifying only the SHA-1, and CGit
provides links to do so, for example if a tag points directly at a blob.
In this case the path_items structure is never used, but creating it
still causes strlen to be run on a null pointer.  Fix this.

This error was introduced by commit c1633c6 (Update git to v1.7.6.5 -
2013-03-02).

Signed-off-by: John Keeping <john@keeping.me.uk>