Return None if calibration matrix is missing

Previously, we raised an exception (and thus HTTP error
code) if the CSM matrix was requested and not yet set.
Now, we just return None, and I've added a (not-network-exposed)
function that will raise an HTTPException if it's missing.

src/openflexure_microscope_server/things/camera_stage_mapping.py

@@ -175,7 +175,7 @@ class CameraStageMapper(Thing):
         return data
 
     @thing_property
-    def image_to_stage_displacement_matrix(self) -> List[List[float]]:  # 2x2 integer array
+    def image_to_stage_displacement_matrix(self) -> Optional[List[List[float]]]:  # 2x2 integer array
         """A 2x2 matrix that converts displacement in image coordinates to stage coordinates.
         
         Note that this matrix is defined using "matrix coordinates", i.e. image coordinates
@@ -195,8 +195,13 @@ class CameraStageMapper(Thing):
         """
         displacement_matrix = self.thing_settings.get("image_to_stage_displacement")
         if not displacement_matrix:
-            raise CSMUncalibratedError()
+            return None
         return np.array(displacement_matrix).tolist()
+    
+    def assert_calibrated(self):
+        """Raise an exception if the image_to_stage_displacement matrix is not set"""
+        if self.image_to_stage_displacement_matrix is None:
+            raise CSMUncalibratedError()
 
     @thing_property
     def last_calibration(self) -> Optional[Dict]:
@@ -222,6 +227,7 @@ class CameraStageMapper(Thing):
         and `y` to the shorter one. Checking what shape your chosen toolkit reports for
         an image usually helps resolve any ambiguity.
         """
+        self.assert_calibrated()
         relative_move: np.ndarray = np.dot(
             np.array([y, x]),
             np.array(self.image_to_stage_displacement_matrix)
@@ -232,12 +238,9 @@ class CameraStageMapper(Thing):
     def thing_state(self) -> dict:
         """Summary metadata describing the current state of the Thing"""
         state: dict[str, Any] = {}
-        try:
-            state["image_to_stage_displacement_matrix"] = (
-                self.image_to_stage_displacement_matrix
-            )
-        except ValueError:
-            pass  # If it can't be retrieved, we'll just return an empty dict
+        state["image_to_stage_displacement_matrix"] = (
+            self.image_to_stage_displacement_matrix
+        )
         if self.last_calibration:
             try:
                 state["image_resolution"] = self.last_calibration["image_resolution"]