BF: make code not puke if there is no encodingFormat in asset metadata

[yarikoptic]

Although it is not clear how that can happen since it is not optional, worth

• [ ] checking which assets lack it and why

Closes #183

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (845cb17) 97.71% compared to head (5168ffd) 97.71%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #184 +/- ## ======================================= Coverage 97.71% 97.71% ======================================= Files 17 17 Lines 1751 1751 ======================================= Hits 1711 1711 Misses 40 40 ``` | Flag | Coverage Δ | | |---|---|---| | unittests | `97.71% <100.00%> (ø)` | | 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=dandi#carryforward-flags-in-the-pull-request-comment) to find out more. | [Impacted Files](https://app.codecov.io/gh/dandi/dandi-schema/pull/184?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dandi) | Coverage Δ | | |---|---|---| | [dandischema/metadata.py](https://app.codecov.io/gh/dandi/dandi-schema/pull/184?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dandi#diff-ZGFuZGlzY2hlbWEvbWV0YWRhdGEucHk=) | `96.63% <100.00%> (ø)` | |

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[satra]

closing in favor of https://github.com/dandi/dandi-archive/pull/1629