fix: resolves all possible warnings

FitConnect/FitConnectClient.cs

@@ -229,9 +229,12 @@ public abstract class FitConnectClient : IFitConnectClient {
 
     protected virtual async Task<ICollection<ValidationError>> CheckDataSchemaAsync(
         Submission submission) {
-        var jSchema = await JsonSchema.FromUrlAsync(submission.Metadata
-            ?.ContentStructure.Data.SubmissionSchema.SchemaUri.ToString());
-        return jSchema.Validate(submission.Data);
+        var url = submission.Metadata
+            ?.ContentStructure.Data.SubmissionSchema.SchemaUri?.ToString();
+        if (url == null)
+            return new List<ValidationError>();
+        var jSchema = await JsonSchema.FromUrlAsync(url);
+        return submission.Data == null ? new List<ValidationError>() : jSchema.Validate(submission.Data);
     }
 
     /// <summary>

FitConnect/Subscriber.cs

@@ -271,6 +271,9 @@ public class Subscriber : FitConnectClient, IReceivedSubmission,
         var submissionAuthentication =
             new AuthenticationTags(dataSignature, metadataSignature, attachmentSignatures);
 
+        if (metadata == null || dataString == null)
+            throw new FitConnectSubscriberException("Metadata or data is null");
+
         return new ReceivedReply(reply.ReplyId, reply.CaseId, metadata, dataString, decryptedAttachments,
             submissionAuthentication);
     }

JwkGenerator/JsonWebKey.cs

@@ -6,21 +6,22 @@ using JsonSerializer = System.Text.Json.JsonSerializer;
 namespace JwkGenerator;
 
 public class JsonWebKey {
-    public string Kty { get; set; }
-    public string Alg { get; set; }
+    public string? Kty { get; set; }
+    public string? Alg { get; set; }
     
     [JsonPropertyName("key_ops")]
     [JsonProperty("key_ops")]
-    public string[] KeyOps { get; set; }
-    public string E { get; set; }
-    public string N { get; set; }
-    public string D { get; set; }
-    public string P { get; set; }
-    public string Q { get; set; }
-    public string Dp { get; set; }
-    public string Dq { get; set; }
-    public string Qi { get; set; }
-    public string Kid { get; set; }
+    public string[]? KeyOps { get; set; }
+    public string? E { get; set; }
+    public string? N { get; set; }
+    public string? D { get; set; }
+    public string? P { get; set; }
+    public string? Q { get; set; }
+    public string? Dp { get; set; }
+    public string? Dq { get; set; }
+    public string? Qi { get; set; }
+    public string? Kid { get; set; }
+
     public static JsonWebKey Deserialize(string publicKey) => JsonSerializer.Deserialize<JsonWebKey>(publicKey) ?? throw new ArgumentNullException(nameof(publicKey));
     public string ToJson() {
         var jsonSerializerOptions = new JsonSerializerOptions() {