pure: fix index parsing on empty repositories

mercurial/pure/parsers.py

@@ -56,6 +56,7 b' def parse_index2(data, inline):'
             n += 1
             off += s
 
+    if index:
     e = list(index[0])
     type = gettype(e[0])
     e[0] = offset_type(0, type)