Instance input WikiStream module should send events even for what is the default from XWiki point of view

Even if it's the default from XWikiDocument point of view it may not be the case for the other modules. For example having an empty revision comment does not mean there is no revision comment (it just mean that XWikiDocument does not make any different between empty comment and no comment). From purely technical point of view there is a comment and it's empty String so it should be sent as it is. Otherwise it mean that roundtripping with XWikiDocument won't work for a module that does not have the same defaults.