Don't generate directory for a zero size tile that is skipped

[zimeon]

Pointed out by @jgreidy, example:

(py3)simeon@RottenApple iiif>wget http://stor.artstor.org/stor/1c7af029-cb44-48e2-ae10-0968a6f28dae
...
(py3)simeon@RottenApple iiif>./iiif_static.py --dst /tmp/iiif 1c7af029-cb44-48e2-ae10-0968a6f28dae
iiif_static.py: source file: 1c7af029-cb44-48e2-ae10-0968a6f28dae
iiif.static: Output directory /tmp/iiif/1c7af029-cb44-48e2-ae10-0968a6f28dae already exists, adding/updating files
... tiles ...
iiif.static: /tmp/iiif / 1c7af029-cb44-48e2-ae10-0968a6f28dae/full/351,/0/default.jpg
iiif.static: /tmp/iiif / 1c7af029-cb44-48e2-ae10-0968a6f28dae/full/351,168 -> 1c7af029-cb44-48e2-ae10-0968a6f28dae/full/351,
iiif.static: /tmp/iiif / 1c7af029-cb44-48e2-ae10-0968a6f28dae/full/175,/0/default.jpg
iiif.static: /tmp/iiif / 1c7af029-cb44-48e2-ae10-0968a6f28dae/full/175,84 -> 1c7af029-cb44-48e2-ae10-0968a6f28dae/full/175,
iiif.static: /tmp/iiif / 1c7af029-cb44-48e2-ae10-0968a6f28dae/full/88,/0/default.jpg
iiif.static: /tmp/iiif / 1c7af029-cb44-48e2-ae10-0968a6f28dae/full/88,42 -> 1c7af029-cb44-48e2-ae10-0968a6f28dae/full/88,
iiif.static: /tmp/iiif / 1c7af029-cb44-48e2-ae10-0968a6f28dae/full/44,/0/default.jpg
iiif.static: /tmp/iiif / 1c7af029-cb44-48e2-ae10-0968a6f28dae/full/44,21 -> 1c7af029-cb44-48e2-ae10-0968a6f28dae/full/44,
iiif.static: /tmp/iiif / 1c7af029-cb44-48e2-ae10-0968a6f28dae/full/22,/0/default.jpg
iiif.static: /tmp/iiif / 1c7af029-cb44-48e2-ae10-0968a6f28dae/full/22,10 -> 1c7af029-cb44-48e2-ae10-0968a6f28dae/full/22,
iiif.static: /tmp/iiif / 1c7af029-cb44-48e2-ae10-0968a6f28dae/full/11,/0/default.jpg
iiif.static: /tmp/iiif / 1c7af029-cb44-48e2-ae10-0968a6f28dae/full/11,5 -> 1c7af029-cb44-48e2-ae10-0968a6f28dae/full/11,
iiif.static: /tmp/iiif / 1c7af029-cb44-48e2-ae10-0968a6f28dae/full/5,/0/default.jpg
iiif.static: /tmp/iiif / 1c7af029-cb44-48e2-ae10-0968a6f28dae/full/5,3 -> 1c7af029-cb44-48e2-ae10-0968a6f28dae/full/5,
iiif.static: /tmp/iiif / 1c7af029-cb44-48e2-ae10-0968a6f28dae/full/3,/0/default.jpg
iiif.static: /tmp/iiif / 1c7af029-cb44-48e2-ae10-0968a6f28dae/full/3,1 -> 1c7af029-cb44-48e2-ae10-0968a6f28dae/full/3,
iiif.static: /tmp/iiif / 1c7af029-cb44-48e2-ae10-0968a6f28dae/full/1,/0/default.jpg - zero size, skipped
iiif.static: /tmp/iiif / 1c7af029-cb44-48e2-ae10-0968a6f28dae/info.json

where 1, was skipped, but still there is a dir:

(py3)simeon@RottenApple iiif>tree /tmp/iiif/1c7af029-cb44-48e2-ae10-0968a6f28dae/full
/tmp/iiif/1c7af029-cb44-48e2-ae10-0968a6f28dae/full
├── 1,
│   └── 0
├── 11,
│   └── 0
│       └── default.jpg
├── 11,5 -> 11,
├── 175,
│   └── 0
│       └── default.jpg
├── 175,84 -> 175,
├── 22,
│   └── 0
│       └── default.jpg
├── 22,10 -> 22,
├── 3,
│   └── 0
│       └── default.jpg
├── 3,1 -> 3,
├── 351,
│   └── 0
│       └── default.jpg
├── 351,168 -> 351,
├── 44,
│   └── 0
│       └── default.jpg
├── 44,21 -> 44,
├── 5,
│   └── 0
│       └── default.jpg
├── 5,3 -> 5,
├── 88,
│   └── 0
│       └── default.jpg
└── 88,42 -> 88,

26 directories, 8 files