Giving a page a title containing only spaces results in an uneditable page with no title

If a user saves a page with only a space in the title, the page sometimes saves, resulting in a page with a bad database entry that cannot be changed. The title will print as $helper.page.title, and if you edit the page, the title is blank. If you try to add a title and change the page, you get a system error.

This is related to https://support.atlassian.com/browse/CSP-48567 .

(I initially thought this happened every time – a user reported they had done this, and when I tested it it worked, but I've since found that a lot of the time it doesn't work – a page with only spaces in the title usually will not save. Blog pages with just a space do, however, seem to save reliably each time, giving a reproducable case.)