Fix undefined offset in updateOneSearchIndexConfig

isset( $wgCirrusSearchReplicas[ $this->indexType ] ) returns true
if $wgCirrusSearchReplicas is a string, so we need to also
check type of $wgCirrusSearchReplicas.

Bug: 72888
Change-Id: I1d8c13cbc2dd5a34b0e5b877a046d314fdefc236