Handle when the slicemap option is an empty string

In the cmdtable for the convert extension, the default value for splicefile is
empty string, while mapfile (the class that reads splicemaps) expects either a
real path or None. This patch changes mapfile to expect a real path or logical
false (False, None, empty string, etc.)

hgext/convert/common.py

@@ -325,7 +325,7 b' class mapfile(dict):'
         self._read()
 
     def _read(self):
-        if self.path is None:
+        if not self.path:
             return
         try:
             fp = open(self.path, 'r')