Failure to add an attachment when the max upload size xproperty is empty

To reproduce:

• Install XS 15.4 hsqldb+jetty
• Edit the "max upload size property" and empty it: http://localhost:8080/xwiki/bin/edit/XWiki/XWikiPreferences?editor=object (search for "Maximum Upload Size", it should be "107374182400" by default)
• Save
• Go to a page and try to upload an attachment. You should get an error such as: