BassClient: Fix not putting empty metadata buffer on the list

The list size has to match the number of subgroups otherwise we would
not be able to connect each subgroup with it's metadata if one of
the subgroups has an empty metadata.

Bug: 150670922
Sponsor: jpawlowski@
Test: tested manually
Change-Id: I7a2a190bc9f8795eab299cece0af0fd5f6e792f0