[BUG] Empty `CertificatePolicy.EnhancedKeyUsage` skips serialization

[MindaugasButkus]

Library name and version

Azure.Security.KeyVault.Certificates 4.8.0

Describe the bug

Using the library there is no way to create a certificate that would not have any EKUs, while using the Azure Portal UI or Rest API directly it is possible.

Library skips serialization of the CertificatePolicy.EnhancedKeyUsage property if it is null or empty (code).

null and empty array EKU in the KeyVault service are interpreted differently. In the service, null (or missing json property) results in a default of 1.3.6.1.5.5.7.3.1 and 1.3.6.1.5.5.7.3.2 EKUs getting added to the certificate. Empty array results in no EKUs. With Azure.Security.KeyVault.Certificates library both of these cases result in 1.3.6.1.5.5.7.3.1 and 1.3.6.1.5.5.7.3.2 EKUs on certificate.

Expected behavior

Having empty list in CertificatePolicy.EnhancedKeyUsage should result in no EKUs on the certificate.
Assigning null to CertificatePolicy.EnhancedKeyUsage should result in default list of EKUs (as defined by the KeyVault Rest API).

Actual behavior

Clearing the list in CertificatePolicy.EnhancedKeyUsage and creating a certificate with it results in a certificate with 2 EKUs - 1.3.6.1.5.5.7.3.1 and 1.3.6.1.5.5.7.3.2.

Reproduction Steps

var client = new CertificateClient(keyVaultUri, credentials);

var newPolicy = new CertificatePolicy(WellKnownIssuerNames.Self, "CN=TestSubject");
newPolicy.EnhancedKeyUsage.Clear();

var certificateOperation = await client.StartCreateCertificateAsync("TestCertificate", newPolicy);
await certificateOperation.WaitForCompletionAsync();

Debug.Assert(certificateOperation.Value.Policy.EnhancedKeyUsage.Count == 0); // Fails

Environment

No response

[github-actions]

Thanks for the feedback! We are routing this to the appropriate team for follow-up. cc @adarce @alukovenko-ms @cheathamb36 @chlowell @dhanvikapila @dragav @fponzi-ms @FRL200 @jackrichins @kasturi3k @ksemenov-gh @kurtzeborn @mancycakv @marekkarbarz @melissamserv @ncamillucci @osmuller @rahulalapati43 @seanbamsft @sebansal @vakuncha @vickm.