prevent null value serialization in JwtSigner (#331)

Signed-off-by: Johannes Tuerk <johannes.tuerk@lissi.id>

Author: JoTiTu

src/WalletFramework.SdJwtVc/Services/SdJwtVcHolderService/SdJwtSigner.cs

@@ -63,8 +63,13 @@ public async Task<string> GenerateKbProofOfPossessionAsync(
 
     public async Task<string> CreateSignedJwt(object header, object payload, KeyId keyId)
     {
-        var encodedHeader = Base64UrlEncoder.Encode(JsonConvert.SerializeObject(header));
-        var encodedPayload = Base64UrlEncoder.Encode(JsonConvert.SerializeObject(payload));
+        var serializerSettings = new JsonSerializerSettings
+        {
+            NullValueHandling = NullValueHandling.Ignore
+        };
+        
+        var encodedHeader = Base64UrlEncoder.Encode(JsonConvert.SerializeObject(header, serializerSettings));
+        var encodedPayload = Base64UrlEncoder.Encode(JsonConvert.SerializeObject(payload, serializerSettings));
 
         var dataToSign = encodedHeader + "." + encodedPayload;
         var signature = await keyStore.Sign(keyId, Encoding.UTF8.GetBytes(dataToSign));