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While at the CTAP layer, the excludeList parameter is optional, omitting
it is equivalent to sending an empty array. Furthermore, an absent
PublicKeyCredentialCreationOptions.excludeCredentials at the Blink layer
is translated into an empty list at the Authenticator mojo interface
(where the corresponding field is non-optional), which demonstrates that
we don't care about the distinction anyway.

Note that while previously, a present-but-empty CTAP excludeList was
marshaled into an empty CBOR Array (inside `AsCTAPRequestValuePair(const
CtapMakeCredentialRequest& request)`), this CL changes it to be omitted
instead.

Change-Id: I8cb72be56351057766c83eb097f9c780fb1567ba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1867385Reviewed-by: Adam Langley <agl@chromium.org>
Commit-Queue: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/master@{#707955}