convert: deal with empty splicemap path (issue3311)

hgext/convert/common.py

@@ -418,6 +418,8 b' class mapfile(dict):'
 
 def parsesplicemap(path):
     """Parse a splicemap, return a child/parents dictionary."""
+    if not path:
+        return {}
     m = {}
     try:
         fp = open(path, 'r')