Make it possible to set an empty figure caption in inline mode

Steps to reproduce

• In inline mode, insert image with the caption option activated
• Edit caption, remove all text and spaces from there
• Switch to source mode

Actual result

The image caption contains one space character (see example below) while an empty caption would fulfill more closely the actual intent. This space character is rendered as an empty paragraph, which means this behaviour prevents from having an empty figcaption block, hence the figcaption block has always a height, which caption custom styling more trickier.

[[ >>image:xwiki.png]]