ensure loader fills image swiper, and only display loader when replay data is nullish, not when it's loaded but there's no image data

[emma-sg]

Closes #1818

• Refactors image renderers, and changes the condition for which the loading indicator is displayed from "<data>.blobUrl exists and has a truthey value" to "<data> is not nullish"
• Adds an additional class config to the shared QA loading indicator, and uses it in the image slider view to add a background and aspect ratio to the loading indicator's container so that it takes up the space it should and has an opaque background

Before	After