Page without background media isn't shown as drop target

[miina]

Bug Description

When the page doesn't have background media assigned, it is not displayed as a drop target.

Expected Behaviour

Page should be a drop target even without background media.

Steps to Reproduce

1. Add an Image to a Page or just use an Image directly from the Library
2. Drag the Image
3. See the Page not being indicated as a drop target.

Screenshots

With background -- works as expected (except for the blue lines remaining post-drop):

Without background:

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Acceptance Criteria

QA Instructions

[miina]

Copying previous discussion from #552:

(Miina): Currently, the Page itself isn't displayng as a drop target if there is no previous background Media, is that intended?

(Dima): Page should display drop target regardless. For all intents and purposes we need to treat the page w/o background as a page with an "empty" background. We did the same for layers. If we can find a way to do this systematically for most of features, great. If not, let's just find a way to do this for DND specifically. But in particular, it means we should avoid showing two outlines when background is available.