do not display "None" for missing PDB ligands for missing compounds

Former-commit-id: 8f2279737984ab478df8807b53180a057250689c

ippisite/ippidb/templates/compound_t_list.html

@@ -18,7 +18,7 @@
         <th scope="col">{{ compound.common_name|default_if_none:"-" }}</th>
         <th scope="col">{{ compound.molecular_weight}}  g/mol</th>
         <th scope="col">{{ compound.a_log_p }}</th>
-        <th scope="col">{% for pdb_id in compound.compound_action_pdb_ids %}{{ pdb_id }}{% endfor %}</th>
+        <th scope="col">{% for pdb_id in compound.compound_action_pdb_ids %}{{ pdb_id | default_if_none:""}}{% endfor %}</th>
         <th scope="col"> ? </th>
         <th scope="col"> ? </th>
         <th scope="col">{{ compound.biblio_refs|length }}</th>