fix: handle missing keys in label mapping

[Samk13]

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Description

I did not digg too deep into this, probablly there is a better way to fix this, but this solve the issue and the error is gone.

• when deleting a funder with API will break the labels
  and we get 500 in UI
• closes: Facet label mapping crashes with KeyError when label_map lacks a bucket key (missing fallback in get_labelled_values) invenio-app-rdm#3257

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