Filtering out licenses with a nil spdx_identifier

What does this MR do and why?

Hotix: filtering out licenses with a nil spdx_identifier

Contributes to: https://gitlab.com/gitlab-org/gitlab/-/issues/505623

We are seeing errors originating from licence entries without a valid spdx_identifier field. The failure occurs when we attempt to compare fields to sort and we compare a nil with a string. This change filters those entries out.

Changelog: fixed

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