Allow [[Special:SetSiteLink//enwiki]]

This allows
https://www.wikidata.org/wiki/Special:SetSiteLink//dewiki
with an empty string as the first parameter, similar to
https://www.wikidata.org/wiki/Special:SetLabel//en,
SetDescription and SetAliases which all already allow this.

Change-Id: I8297f9382cb8b6509a7f3507f6807b3c74ea0184
Merge "Allow [[Special:SetSiteLink//enwiki]]"