fix kb-jwt payload if value is null

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

Author: JoTiTu

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

@@ -31,15 +31,19 @@ public async Task<string> GenerateKbProofOfPossessionAsync(KeyId keyId, string a
             }
         }
 
-        var payload = new
+        var payload = new Dictionary<string, object>
         {
-            aud = audience,
-            nonce,
-            iat = DateTimeOffset.UtcNow.ToUnixTimeSeconds(),
-            sd_hash = sdHash,
-            iss = clientId
+            { "aud", audience },
+            { "nonce", nonce },
+            { "iat" , DateTimeOffset.UtcNow.ToUnixTimeSeconds() }
         };
 
+        if (!sdHash.IsNullOrEmpty())
+            payload["sd_hash"] = sdHash!;
+        
+        if (!clientId.IsNullOrEmpty())
+            payload["iss"] = clientId!;
+
         return await CreateSignedJwt(header, payload, keyId);
     }