getRenderedTitle should fallback on the parent space name when a WebHome page has the title empty

If MySpace.WebHome has the title empty then

$xwiki.getDocument('MySpace.WebHome').plainTitle

should output "MySpace". The output is currently "WebHome" (the page name) which is technical and confuses the simple users.

Note that when we create a space the title is prefilled with the space name so if you leave the title as is then you don't see "WebHome" after saving the page. But if you clear the title field then you get to see "WebHome".