Standalone Sprites in the Body and Buttons of a Dialog aren't rendered

Description
If there is a sprite in either the body or in a button in a dialog, it won’t show up unless there is some text in the same line as the sprite.

Steps to reproduce the issue
Run each of the following commands in command blocks separately, and observe the difference.
(The second command is the same as the first, but without the periods)

dialog show @p {type:"minecraft:multi_action",title:{sprite:"block/emerald_block"},body:[{type:"minecraft:plain_message",contents:{sprite:"block/iron_block",extra:["."]}},{type:"minecraft:plain_message",contents:[{text:"."},{sprite:"minecraft:block/diamond_block"}]}],actions:[{label:{sprite:"block/gold_block",extra:["."]}}]}
dialog show @p {type:"minecraft:multi_action",title:{sprite:"block/emerald_block"},body:[{type:"minecraft:plain_message",contents:{sprite:"block/iron_block",extra:[""]}},{type:"minecraft:plain_message",contents:[{text:""},{sprite:"minecraft:block/diamond_block"}]}],actions:[{label:{sprite:"block/gold_block",extra:[""]}}]}

Expected Result

Both commands should show all of the sprites

Actual Result

Only the first command shows all of the sprites, because there is text before or after each sprite, whereas in the second command the text is removed, so none of the sprites show.