[FIX] allow saving diagrams online in wiki pages

Diagrams could not be saved online in wiki pages due to the use of the "??" (or null coalescing) operator on a Boolean variable(galleryId) in plugin diagram. This operator only returns true if the object is equal to null otherwise it returns false. So as the diagram plugin saves the content of the file in the body of the plugin if this parameter does not exist, I had to make sure that if the variable(galleryId) passed in parameter does not exist, null value will be sent instead of false. Now diagrams can be saved online in wiki pages.