fix(previewer): media config not initialized

[BrayanDSO]

Fixes

• Fixes Previewer: kotlin.UninitializedPropertyAccessException: lateinit property config has not been initialized #19729

Approach

used a nullable config and empty av tags lists instead of lateinit properties.

This avoids crashes. There's still the rare chance of the previewer requesting a sound before it's loaded, which would result in playing nothing. Making that foolproof requires a bigger refactor, which I intend to do later

How Has This Been Tested?

Flaky hammer™: https://github.com/BrayanDSO/Anki-Android/actions/runs/21093913315

Checklist

Please, go through these checks before submitting the PR.

• You have a descriptive commit message with a short title (first line, max 50 chars).
• You have commented your code, particularly in hard-to-understand areas
• You have performed a self-review of your own code
• UI changes: include screenshots of all affected screens (in particular showing any new or changed strings)
• UI Changes: You have tested your change using the Google Accessibility Scanner

[BrayanDSO]

Asking for some attention on the condition fixes of AbstractFlashcardViewer

[mikehardy]

conditional changes look right, going with empty as default init seems fine as a crash-fix

Flaky hammer™: https://github.com/BrayanDSO/Anki-Android/actions/runs/21093913315

😍