While all of the following are valid in the model:
1. <mwBlockImage></mwBlockImage>
Image with no caption. Must use the media dialog to insert one.
2. <mwBlockImage><mwImageCaption></mwImageCaption></mwBlockImage>
Image with empty caption. There is a slug to insert a paragraph.
3. <mwBlockImage><mwImageCaption><paragraph></paragraph></mwImageCaption></mwBlockImage>
Image with caption with empty paragraph. Nice and intuitive!
(Same for <mwGalleryImage> / <mwGalleryImageCaption>.)
The third option is the most convenient for the user. We should always
generate that when converting documents from HTML and from the editing
tools (MWGalleryDialog, MWMediaDialog/MWImageModel).
Previously, the editing tools generated option 2 if no caption text
was entered, and the converter generated option 2 if there was no
caption node or if it was empty. Curiously, option 1 was never used.
Wikitext for manual testing:
```
[[File:Foo.png|thumb]]
[[File:Foo.png|thumb|]]
[[File:Foo.png|thumb|Caption]]
<gallery mode="packed">
File:Foo.png
File:Foo.png|
File:Foo.png|Caption
</gallery>
```
Bug: T200387
Change-Id: Ie82fb339f6bd8ae1b289235bf5402490722d9a7c
