guard against cf_name.canonical_units being None in get_nc_attrs

Author: DocOtak

src/cchdo/params/core.py

@@ -274,7 +274,8 @@ def get_nc_attrs(self, error=False):
                 standard_name = f"{standard_name} standard_error"
 
             attrs["standard_name"] = standard_name
-            attrs["units"] = cf_name.canonical_units
+            if cf_name.canonical_units is not None:
+                attrs["units"] = cf_name.canonical_units
 
         if self.cf_unit is not None:
             attrs["units"] = self.cf_unit