Omit empty fields from DeviceCredential resources

[codingllama]

Omits, from the resource representation of devices, fields that are always empty in some cases.

DeviceAttestationType is particular in the sense that it tends to show up as "unspecified", while other enums are never unspecified in valid devices (OSType, EnrollStatus), or tend to have their entire parent message/struct nil when they would be unspecified (DeviceOrigin).

-before/+after:

tctl get device/foo
kind: device
metadata:
  name: 9c39701e-9f19-4766-bc0c-1241d08ce649
spec:
  asset_tag: foo
  create_time: "2023-06-13T17:47:51.169251Z"
  credential:
-   device_attestation_type: unspecified
    id: ae2d978c-fee8-419d-a2e6-a5dd0a00c4b8
    public_key_der: MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEg1iznap59moe9gDCySvK1dYkenZBgRdif1No8u4sjlhp7qyRjMxSoFv8wytBCZNSJRRZ55prKVlMEjxcdFtJlQ==
-   tpm_ak_public: null
-   tpm_ekcert_serial: ""
  # so on ...
version: v1

https://github.com/gravitational/teleport.e/issues/827

[codingllama]

@strideynet, did I tag the correct issue in the description? Feel free to edit if I didn't.

[codingllama]

Friendly ping @avatus @nklaassen ?

[public-teleport-github-review-bot]

@codingllama See the table below for backport results.

Branch	Result
branch/v13	Create PR