Handle empty list of repositories

modbase_pdb_to_cif.py

@@ -126,7 +126,7 @@ def __call__(self, asym_id, entity_id, seq_id, pdb_seq_num, pdb_ins_code,
 class Repository:
     """Point to directory/ies containing a mirror of PDB in mmCIF format"""
     def __init__(self, topdirs):
-        self.topdirs = topdirs
+        self.topdirs = topdirs or []
 
     def open_mmcif(self, pdb_code):
         """Given a PDB code, return a file handle to the corresponding mmCIF"""