Revert "Use default value instead of validator"

This reverts commit 8e2bb73.

src/hyperion/parameters/rotation.py

@@ -14,7 +14,8 @@
 from dodal.devices.zebra import (
     RotationDirection,
 )
-from pydantic import Field, root_validator
+from dodal.log import LOGGER
+from pydantic import Field, root_validator, validator
 from scanspec.core import AxesPoints
 from scanspec.core import Path as ScanPath
 from scanspec.specs import Line
@@ -50,9 +51,6 @@ class RotationExperiment(DiffractionExperimentWithSample):
     ispyb_experiment_type: IspybExperimentType = Field(
         default=IspybExperimentType.ROTATION
     )
-    selected_aperture: AperturePositionGDANames | None = Field(
-        default=DEFAULT_APERTURE_POSITION
-    )
 
     def _detector_params(self, omega_start_deg: float):
         self.det_dist_to_beam_converter_path = (
@@ -83,6 +81,18 @@ def _detector_params(self, omega_start_deg: float):
             **optional_args,
         )
 
+    @validator("selected_aperture")
+    def _set_default_aperture_position(
+        cls, aperture_position: AperturePositionGDANames | None
+    ):
+        if not aperture_position:
+            LOGGER.warn(
+                f"No aperture position selected. Defaulting to {DEFAULT_APERTURE_POSITION}"
+            )
+            return DEFAULT_APERTURE_POSITION
+        else:
+            return aperture_position
+
 
 class RotationScan(WithScan, RotationScanPerSweep, RotationExperiment):
     @property