Support profile pages missing a 'name' should adapt their page titles

[jonoaldersonwp]

Profiles created by users in the past (pre ~2010?) sometimes omit a 'name'. This results in the page title 'breaking'.

E.g., all of the following have a missing name, and a page title of 's Profile | WordPress.org:
​https://wordpress.org/support/users/withunveiledface/
​https://wordpress.org/support/users/sakyant/
​https://wordpress.org/support/users/guidowenzl/
​https://wordpress.org/support/users/voodoolove1/

When the name parameter is missing, the <title> should use the handle instead.

E.g., the title of ​https://wordpress.org/support/users/withunveiledface/ should be @withunveiledface's Profile | WordPress.org.

[jonoaldersonwp]

Upgraded severity; there are thousands of these :D

[dd32]

In 10238:

Add a MU plugin to fill in users empty display_name fields when possible.

Some users on WordPress.org have empty display_name fields due to bugs in sanitization of user profile saving.

Fixes #5416.
See #4644.