[Bmon] Ignore unmapped channels in online unpacker

=> Due to noisy unconnected channel not being masked in HW during mCBM 2024 data taking (and probably 2022 one)

Part of the stack of non-CI commits in !1874 (merged), separated to make it more visible/obvious and get it in without waiting for resolution of the online logger troubles

added BugFix Online Reconstruction mCBM labels

requested review from @d.smith

assigned to @p.-a.loizeau

Dear @d.smith, @fweig,

you have been identified as code owner of at least one file which was changed with this merge request.

Please check the changes and approve them or request changes.

added CodeOwners label

mentioned in merge request !1874 (merged)

approved this merge request

merged