Fixed issue with thumbnails not showing for assets missing dc:format

[davidjgonzalez]

Fixed issue in AssetDelivery dispatcher where it would NPE if the dc:format property was not set on an asset (which appears to be the case for OFT files).

Also, added JS fallback support for imgs that dont load (to use the missing image, cliend-side).

Description

Related Issue

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [ ] I have signed the Adobe Open Source CLA.
• [ ] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 28.19%. Comparing base (4a0ff9b) to head (64747ab). Report is 31 commits behind head on develop.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #1159 +/- ## ============================================= + Coverage 27.79% 28.19% +0.40% - Complexity 538 552 +14 ============================================= Files 180 182 +2 Lines 4684 4738 +54 Branches 662 673 +11 ============================================= + Hits 1302 1336 +34 - Misses 3196 3213 +17 - Partials 186 189 +3 ``` | [Flag](https://app.codecov.io/gh/adobe/asset-share-commons/pull/1159/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=adobe) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/adobe/asset-share-commons/pull/1159/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=adobe) | `28.19% <100.00%> (+0.40%)` | :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=adobe#carryforward-flags-in-the-pull-request-comment) to find out more.

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