Check for existence of iw_local in Interwiki::getAllPrefixes

Interwiki::getAllPrefixes only sets the iw_local member in a response
row if $local is set to a non-null value. Under certain error
reporting conditions, the unqualified access to the array key leads to
warnings such as "message repeated 1471 times: [ #012Notice: Undefined
index: iw_trans in ...".

Change-Id: Ibf434dc9c362984cad76ae6bdb29de826974d81e