Fix breaking old stair generation tests after f52aafd

Before f52aafd default `(0,0)` meant that there are no custom tread runs, but after the change it actually means that runs for first and last treads are 0. So default value should be adjusted.
Added explicit `None` value for clarity.

src/bonsai/bonsai/bim/module/model/prop.py

@@ -608,16 +608,18 @@ class BIMStairProperties(PropertyGroup):
             }
             stair_kwargs.update(generic_props)
 
+        non_si_units_props = self.non_si_units_props
         # If locked, use tread_run for both first and last treads
         if self.custom_tread_lock:
-            stair_kwargs["custom_first_last_tread_run"] = (self.tread_run, self.tread_run)
+            non_si_units_props += ("custom_first_last_tread_run",)
+            stair_kwargs["custom_first_last_tread_run"] = (None, None)
         else:
             stair_kwargs["custom_first_last_tread_run"] = self.custom_first_last_tread_run
 
         if not convert_to_project_units:
             return stair_kwargs
 
-        stair_kwargs = tool.Model.convert_data_to_project_units(stair_kwargs, self.non_si_units_props)
+        stair_kwargs = tool.Model.convert_data_to_project_units(stair_kwargs, non_si_units_props)
         return stair_kwargs
 
     def get_props_kwargs_for_ifc_export(self, convert_to_project_units=False, stair_type=None):

src/bonsai/bonsai/tool/model.py

@@ -1398,7 +1398,7 @@ class Model(bonsai.core.tool.Model):
         has_top_nib=None,
         top_slab_depth=None,
         base_slab_depth=None,
-        custom_first_last_tread_run=(0, 0),
+        custom_first_last_tread_run: Union[tuple[float, float], tuple[None, None]] = (None, None),
         nosing_length=0,
         # CONCRETE GENERIC STAIR ARGUMENTS
         nosing_depth=0,