Declare gdal_proximity to handle nullcheck

opendm/dem/commands.py

@@ -20,6 +20,8 @@ from opendm import log
 from .ground_rectification.rectify import run_rectification
 from . import pdal
 
+gdal_proximity = None
+
 try:
     # GDAL >= 3.3
     from osgeo_utils.gdal_proximity import main as gdal_proximity
@@ -263,4 +265,4 @@ def get_dem_radius_steps(stats_file, steps, resolution, multiplier = 1.0):
     for _ in range(steps - 1):
         radius_steps.append(radius_steps[-1] * math.sqrt(2))
     
-    return radius_steps
+    return radius_steps