fix: iterate only over groups that exist in the datatree

Author: wietzesuijker

src/eopf_geozarr/conversion/geozarr.py

@@ -121,6 +121,8 @@ def setup_datatree_metadata_geozarr_spec_compliant(
     grid_mapping_var_name = "spatial_ref"
 
     for key in groups:
+        if key not in dt:
+            continue
         if not dt[key].data_vars:
             continue