Add type hint in ApiClientInfo + fix test

For compatibility with Ie6bf191 in core, though this
change also works ok with earlier versions of MediaWiki core.

Bug: 65440
Change-Id: Icd9627bc6fec90216d91949492db74b547eba83e
(cherry picked from commit 355c5cf33f2973a320f998ca3d0dcfa0499cded6)