Exclude empty objects during picture migration

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (bd8dca6) 100.00% compared to head (ac95fe4) 100.00%. Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #120 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 13 13 Lines 448 449 +1 ========================================= + Hits 448 449 +1 ``` | Flag | Coverage Δ | | |---|---|---| | celery | `85.30% <100.00%> (+0.03%)` | :arrow_up: | | cleanup | `85.96% <100.00%> (+0.03%)` | :arrow_up: | | dj4.1 | `85.96% <100.00%> (+0.03%)` | :arrow_up: | | dj4.2 | `85.96% <100.00%> (+0.03%)` | :arrow_up: | | dramatiq | `85.30% <100.00%> (+0.03%)` | :arrow_up: | | drf | `93.98% <100.00%> (+0.01%)` | :arrow_up: | | macos-latest | `86.19% <100.00%> (+0.03%)` | :arrow_up: | | py3.10 | `85.96% <100.00%> (+0.03%)` | :arrow_up: | | py3.8 | `85.96% <100.00%> (+0.03%)` | :arrow_up: | | py3.9 | `85.96% <100.00%> (+0.03%)` | :arrow_up: | | ubuntu-latest | `85.96% <100.00%> (+0.03%)` | :arrow_up: | | windows-latest | `78.30% <100.00%> (+0.07%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Johannes+Maron#carryforward-flags-in-the-pull-request-comment) to find out more. | [Files Changed](https://app.codecov.io/gh/codingjoe/django-pictures/pull/120?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Johannes+Maron) | Coverage Δ | | |---|---|---| | [pictures/migrations.py](https://app.codecov.io/gh/codingjoe/django-pictures/pull/120?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Johannes+Maron#diff-cGljdHVyZXMvbWlncmF0aW9ucy5weQ==) | `100.00% <100.00%> (ø)` | |

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