Prior to #1912, height fix up of image events without an `info` in their
content would fail, setting `style.height = null + "px"`.
Now that all thumbnail sizing is done through one path, we can fix the
same problem for all cases (images, stickers, e2e/non-e2e) by handling
images without `info` correctly.
At the bare minimum, we use a null-guard that will make sure an image
without an `info` does not appear in the timeline (as a spinner or
otherwise until loaded). When loaded, we size it like any other image
by using the natural dimensions of the loaded image in place of `info`.
Note that we do not apply the same logic to images that *do* specify an
`info` with `w` and `h` keys. If the aspect ratio of the image does not
match that of the event, we use the one in `info` even when the image
has loaded.
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