Spotted in production: `Blocker must be a local user or a name that cannot be a local user`
Urls look like: `/w/index.php?title=Special:UserLogin&returnto=blahblahblah`
Stacktrace:
```
#0 /srv/mediawiki/php-1.31.0-wmf.11/extensions/GlobalBlocking/includes/GlobalBlock.php(17): Block->setBlocker(User)
#1 /srv/mediawiki/php-1.31.0-wmf.11/extensions/GlobalBlocking/includes/GlobalBlocking.class.php(21): GlobalBlock->__construct(stdClass, array)
#2 /srv/mediawiki/php-1.31.0-wmf.11/extensions/GlobalBlocking/includes/GlobalBlockingHooks.php(87): GlobalBlocking::getUserBlock(User, string)
#3 /srv/mediawiki/php-1.31.0-wmf.11/includes/Hooks.php(177): GlobalBlockingHooks::onUserIsBlockedGlobally(User, string, boolean, NULL)
#4 /srv/mediawiki/php-1.31.0-wmf.11/includes/Hooks.php(205): Hooks::callHook(string, array, array, NULL)
#5 /srv/mediawiki/php-1.31.0-wmf.11/includes/user/User.php(2180): Hooks::run(string, array)
#6 /srv/mediawiki/php-1.31.0-wmf.11/includes/user/PasswordReset.php(262): User->getGlobalBlock()
#7 /srv/mediawiki/php-1.31.0-wmf.11/includes/user/PasswordReset.php(103): PasswordReset->isBlocked(User)
#8 /srv/mediawiki/php-1.31.0-wmf.11/includes/specialpage/LoginSignupSpecialPage.php(1143): PasswordReset->isAllowed(User)
#9 /srv/mediawiki/php-1.31.0-wmf.11/includes/specialpage/LoginSignupSpecialPage.php(845): LoginSignupSpecialPage->getFieldDefinitions(FakeAuthTemplate)
#10 /srv/mediawiki/php-1.31.0-wmf.11/includes/specialpage/AuthManagerSpecialPage.php(652): LoginSignupSpecialPage->onAuthChangeFormFields(array, array, array, string)
#11 /srv/mediawiki/php-1.31.0-wmf.11/includes/specialpage/LoginSignupSpecialPage.php(669): AuthManagerSpecialPage->fieldInfoToFormDescriptor(array, array, string)
#12 /srv/mediawiki/php-1.31.0-wmf.11/includes/specialpage/AuthManagerSpecialPage.php(400): LoginSignupSpecialPage->getAuthForm(array, string)
#13 /srv/mediawiki/php-1.31.0-wmf.11/includes/specialpage/LoginSignupSpecialPage.php(316): AuthManagerSpecialPage->trySubmit()
#14 /srv/mediawiki/php-1.31.0-wmf.11/includes/specialpage/SpecialPage.php(522): LoginSignupSpecialPage->execute(NULL)
#15 /srv/mediawiki/php-1.31.0-wmf.11/includes/specialpage/SpecialPageFactory.php(578): SpecialPage->run(NULL)
#16 /srv/mediawiki/php-1.31.0-wmf.11/includes/MediaWiki.php(287): SpecialPageFactory::executePath(Title, RequestContext)
#17 /srv/mediawiki/php-1.31.0-wmf.11/includes/MediaWiki.php(851): MediaWiki->performRequest()
#18 /srv/mediawiki/php-1.31.0-wmf.11/includes/MediaWiki.php(523): MediaWiki->main()
#19 /srv/mediawiki/php-1.31.0-wmf.11/index.php(43): MediaWiki->run()
#20 /srv/mediawiki/w/index.php(3): include(string)
#21 {main}
```