In the scans indexer, do not convert mm to px when `indexing.refineme…

…nt_protocol.mode==None` (dials#2687)

* Check required column is present before converting mm to px.

This is what is done by the StillsIndexer when refinement_protocol.mode
is None, so do the same here.

newsfragments/2687.bugfix

@@ -0,0 +1,2 @@
+``dials.index``: Avoid mm to px conversion when the ``refinement_protocol``
+is set to do no refinement, as the required data are not available.

src/dials/algorithms/indexing/indexer.py

@@ -783,7 +783,8 @@ def index(self):
                 rotation_matrix_differences(self.refined_experiments.crystals())
             )
 
-        self._xyzcal_mm_to_px(self.refined_experiments, self.refined_reflections)
+        if "xyzcal.mm" in self.refined_reflections:
+            self._xyzcal_mm_to_px(self.refined_experiments, self.refined_reflections)
 
     def _unit_cell_volume_sanity_check(self, original_experiments, refined_experiments):
         # sanity check for unrealistic unit cell volume increase during refinement