fix gallery next and prev navigation

[alecananian]

Simplifies gallery navigation logic and fixes issue with navigating to empty/deleted detail page

[linear]

LAND-1440 Encountered bug while tapping “next” post button in gallery: Right side of assignment cannot be destructured

[mrozanski]

What are the actual steps to reproduce the issue being fixed?
Here's what I tested:

From Newest post

1. Open a gallery (from a mobile size screen) with two posts (photos)
2. Open the first photo in the list (latest uploaded)
3. Expected: no left arrow, and right arrow present (takes me to next photo in the list)
4. Observed: Left arrow present (tapping closes the gallery) and no right arrow

This branch fixes that behavior

From Oldest post

1. Open a gallery (from a mobile size screen) with two posts (photos)
2. Open the last photo in the list (oldest uploaded)
3. Expected: no right arrow, and left arrow present (takes me to previous photo in the list)
4. Observed: both arrows present (tapping right one generates exception)

This branch fixes that behavior too

What else should I test?

[alecananian]

There seems to be some way that a post becomes null, so this PR fixes issues that arise from that by adding the filter .filter(([, v]) => v !== null) that is used elsewhere. I'm not sure the steps to reproduce (may have to do with a deleted post), but you can see the issue by following the path in the ticket and navigating through that gallery.