Add null-conditional operators for safer null handling

Co-authored-by: romanett <7413710+romanett@users.noreply.github.com>

Author: Copilot

Tests/Opc.Ua.Client.Tests/ClientFixture.cs

@@ -516,16 +516,16 @@ private void Session_KeepAlive(ISession session, KeepAliveEventArgs e)
             if (ServiceResult.IsBad(e.Status))
             {
                 // Ignore expected errors during test shutdown to reduce noise in CI logs
-                if (e.Status.StatusCode == StatusCodes.BadServerHalted ||
-                    e.Status.StatusCode == StatusCodes.BadNoCommunication)
+                if (e.Status?.StatusCode == StatusCodes.BadServerHalted ||
+                    e.Status?.StatusCode == StatusCodes.BadNoCommunication)
                 {
                     return;
                 }
 
                 m_logger.LogError(
                     "Session '{SessionName}' keep alive error: {StatusCode}",
                     session.SessionName,
-                    e.Status.ToLongString());
+                    e.Status?.ToLongString());
             }
         }
     }