Virtual keyboard dismisses when focus moves from image/video caption to paragraph

[hypest]

Steps to reproduce:

1. Run the demo app
2. Tap on the image (or video) block to select it
3. Tap in its caption. The virtual keyboard should come up.
4. Tap in a paragraph or heading block
5. Notice the virtual keyboard becoming dismissed while the focus and caret is in the paragraph block

[mchowning]

Also observed this in v1.8.0 (6025fe9).

I did observe that the issue did not always occur (most of these tests were with an image caption, but video captions appeared to behave similarly):

Switching away from image with empty caption

Scenario	Frequency of Issue Occurring
Image had no caption, and no text added to caption	Always
Image had caption, caption was entirely deleted	Usually

Switching away from image with non-empty caption

Scenario	Frequency of Issue Occurring
Had no caption, and text added to caption	Never
Had caption, and caption was not changed	Never
Had caption, caption was entirely deleted, text was added back to caption (all done while not losing focus on the caption)	Infrequently

The fact that having a caption avoids the issue when you delete and re-add the caption (sometimes) was pretty surprising to me. 🤔 Seems like I haven't really isolated the behavior well.

Possibly Related Issue

I also observed the keyboard-dismiss-behavior occur infrequently (was pretty rare though) when returning to the caption:

1. Heading block selected with keyboard open
2. Tap on non-empty caption
3. Observe the keyboard dismissed.

[mchowning]

A bit of additional info for whoever picks this up (in case it's not me): the issue appears to be related to the fact that we do not render the component for an empty caption if the image/video block is not selected. Changing it so that the caption component always renders, even if it's empty, prevents the keyboard from dismissing.