When an MR is merged via the API without `should_remove_source_branch`
set, the branch is always retained, even if `force_remove_source_branch`
is set to true in the MR merge params. This was happening because the
API always passed a `should_remove_source_branch` with a `nil` value,
and `MergeService` interpreted this as `false`.

To fix this, we strip `nil` values from the `merge_params`.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/219991