Fixed "no implicit conversion of nil into String (TypeError)"

This happens if MEDIAWIKI_USER environment variable is not set:

$ bundle exec cucumber
no implicit conversion of nil into String (TypeError)
…tests/browser/features/support/pages/user_page.rb:6:in `+'
…

Change-Id: Id76b4df32f85fa30acacc7641a06eb301f2e9331
Merge "Fixed "no implicit conversion of nil into String (TypeError)""