Fix nullable reference type warnings

Author: toddmeng-db

csharp/src/Drivers/Apache/Spark/SparkHttpConnection.cs

@@ -256,7 +256,7 @@ protected internal override Task<TRowSet> GetRowSetAsync(TGetPrimaryKeysResp res
         private string GetUserAgent()
         {
             // Check if a client has provided a user-agent entry
-            if (Properties.TryGetValue(SparkParameters.UserAgentEntry, out string userAgentEntry) && !string.IsNullOrWhiteSpace(userAgentEntry))
+            if (Properties.TryGetValue(SparkParameters.UserAgentEntry, out string? userAgentEntry) && !string.IsNullOrWhiteSpace(userAgentEntry))
             {
                 return $"{s_baseUserAgent} {userAgentEntry}";
             }

csharp/test/Drivers/Apache/Spark/SparkHttpConnectionUserAgentTest.cs

@@ -123,7 +123,7 @@ private string GetUserAgentFromConnection(Dictionary<string, string> properties)
             }
 
             // Invoke the method
-            return (string)getUserAgentMethod.Invoke(connection, null);
+            return (string)getUserAgentMethod.Invoke(connection, null)!;
         }
     }
 }