The key is not send, the key is not included in the prefetch field so it is absent in the prefetch field

Existing Wording:

The CDS Client MUST NOT send any prefetch template key that it chooses not to satisfy. Similarly, if the CDS Client encounters an error while prefetching any data, the prefetch template key MUST NOT be sent to the CDS Service. If the CDS Client has no data to populate a template prefetch key, the prefetch template key MUST have a value of null. Note that the null result is used rather than a bundle with zero entries to account for the possibility that the prefetch url is a single-resource request.

Proposed Wording:

The CDS Client MUST NOT include any prefetch template key that it chooses not to satisfy. Similarly, if the CDS Client encounters an error while prefetching any data, the prefetch template key MUST NOT be included to the CDS Service. If the CDS Client has no data to populate a template prefetch key, the prefetch template key MUST have a value of null. Note that the null result is used rather than a bundle with zero entries to account for the possibility that the prefetch url is a single-resource request.