Fix invalid array_merge in MergeItems api module

This module inherits directly from ApiBase where
getParamDescription returns false, thus calling it then
array_merge results in a php warning and incomplete
description on the api help page.

Bug: 71469
Change-Id: Ib508355a3720c1c54b95c34aeb2446ac48909b32
Merge "Fix invalid array_merge in MergeItems api module" into wmf/1.25wmf1