Is there an addon that would give me a report of all items that are missing content in a certain field?
The use case here is to find images where the alt text field is empty.
I'm imagining a UI where I can select a content type from a dropdown. Then, the UI populates a list of checkboxes where each checkbox represents a field on this content type. I can then check one or more boxes and click a submit button that would then generate a report of all of the items in the CMS, of that content type, where one or more of the fields I selected are empty.
I haven't seen a generic add-on for this type of report. Generally you won't see something like this since content type models for MediaData and the properties associated with those content types are not out-of-the-box with Optimizely (i.e. it's an implementation task to create those custom content types and properties).
From here, you have some options:
Personally, I would just create a quick scheduled job.
I know this is not what you're asking, but this sounds like an add-on that would treat the symptom (fixing missing fields after-the-fact) and not the problem (preventing missing fields to happen in the first place).
To avoid this problem for future content, I'd use the Validation feature to validate properties on a page, block or form. This ensures that properties that are required, or recommended, to have a value, are filled before the piece of content gets published.