Fix Maintenance::output when called very early
If a maintenance script is called with --help (or without
required args), this is currently failing, as MediaWikiServices
is only loaded after Maintenance::setup is initially called.
Follows 336454104d1249439e3ff97746de69c5e7b4a5f9.
Change-Id: Ied283d362675b6b98bd3144132a240b432998991
(cherry picked from commit 803a29127ce02734ab2dd83fff2cf716c98eedad)