CarPlay: only cache the episode image if it exists

[leandroalonso]

A user contacted us about some episode images not appearing at all.

I was able to reproduce the issue with the podcasts they provided:

• https://pca.st/SeFq
• https://pca.st/eci3r72a
• https://pca.st/5o8rkr8q
• https://pca.st/hpu38ns1
• https://pca.st/R40E
• https://pca.st/cuma9pce
• https://pca.st/lbdkdt3w

This PR changes how we cache images so we don't cache the "default" image.

To test

1. Add some of the podcasts to your account
2. Open the CarPlay simulator
3. Go to Filters > New Releases
4. Check that the images appear correctly

Checklist

• I have considered if this change warrants user-facing release notes and have added them to CHANGELOG.md if necessary.
• I have considered adding unit tests for my changes.
• I have updated (or requested that someone edit) the spreadsheet to reflect any new or changed analytics.

[dangermattic]

	1 Warning
⚠️	This PR is assigned to the milestone 7.80 ❄️. The due date for this milestone has already passed. Please assign it to a milestone with a later deadline or check whether the release for this milestone has already been finished.

Generated by 🚫 Danger

[SergioEstevao]

Great catch!