Log warning and show error on empty username
Historically it seems that if Linker::userLink or friends were passed an
empty username (probably due to an incorrect database entry), they would
produce bogus output, e.g., an <a> with no contents or a link to the
invalid page "User_talk:" or similar.
In b6e1e99bec8d we replaced an occurrence of Title::makeTitle() (no
safety checks!) with creating a TitleValue, which asserts in its
constructor that the title text is not empty. This made such pages fail
an assertion and stop displaying at all.
Now there's a proper check for the error. Such cases will log a
production error and return "(no username available)".