Return $wgSitename as displayname for local repo in filerepoinfo API

displayname is used in user interface texts to tell the user what
is the source of a file so returning null does not make sense
here.

Bug: 64800
Change-Id: Iafbc1a8e63b512d1515215fa1295e5b7434f1861
Merge "Return $wgSitename as displayname for local repo in filerepoinfo API"