refactor: make Sanitize.SanitizeUserInput null-safe

Accept a nullable string and return string.Empty for null input rather
than throwing NullReferenceException. This addresses the root cause of
the test regression introduced when sanitizing missionTask.Id in
AreaPolygonService and removes the need for an inline null check at
the call site.

All other callers pass values that are either guaranteed non-null by
model binding ([Required] DTO properties) or already null-checked
explicitly (UpdateRobotQuery), so the new behaviour is observationally
equivalent for them while removing a sharp edge for future callers.

Author: Christdej

backend/api/Services/AreaPolygonService.cs

@@ -45,9 +45,7 @@ public bool MissionTasksAreInsideAreaPolygon(
                         robotPosition.X,
                         robotPosition.Y,
                         robotPosition.Z,
-                        missionTask.Id is null
-                            ? string.Empty
-                            : Sanitize.SanitizeUserInput(missionTask.Id)
+                        Sanitize.SanitizeUserInput(missionTask.Id)
                     );
                     return false;
                 }

backend/api/Utilities/SanitizeInput.cs

@@ -5,9 +5,9 @@ namespace Api.Utilities
 {
     public static class Sanitize
     {
-        public static string SanitizeUserInput(string inputString)
+        public static string SanitizeUserInput(string? inputString)
         {
-            return inputString.Replace("\n", "").Replace("\r", "");
+            return inputString?.Replace("\n", "").Replace("\r", "") ?? string.Empty;
         }
 
         public static ScheduleMissionQuery SanitizeUserInput(ScheduleMissionQuery inputQuery)