JsonIgnore missing on SignKeyRequest properties

[lukeocamden]

Describe the bug
Serialized SigninKeyRequest can be invalid due to missing [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]

VaultSharp Version
1.13.0.1

Vault Version
1.13.2+ent

Does this work with Vault CLI?
Yes

Sample Code Snippet

var signedKey = await vaultClient.V1.Secrets.SSH.SignKeyAsync(principal,
    new SignKeyRequest
    {
        ValidPrincipals = principal,
        PublicKey = publicKey,
    }, "ssh/whatever");

Exception Details/Stack Trace/Error Message
Permission denied from server.

Any additional info
Hello! In 11af98d you fixed lots of properties so null values would not be sent to Vault API (fixing #312). However I think SignKeyRequest.cs was missed from this change. Is it easy to fix?