fix keyerror when DMD_EXTENSIONS metadata not set

preprocessor/preprocessor/steps/output.py

@@ -15,8 +15,7 @@ def output_step(source_dir: os.PathLike, target_dir: os.PathLike, options: dict=
     driver = gdal.GetDriverByName(frmt)
     if not driver:
         raise ValueError('Unsupported driver %s' % frmt)
-    extension = driver.GetMetadata()['DMD_EXTENSIONS'].split(' ')[0]
-
+    extension = driver.GetMetadata().get('DMD_EXTENSIONS', 'tif').split(' ')[0]
     # warp each individual file
     warped_files = []
     for filename in glob(join(source_dir, '*.tif')):