BigFile: No need to explicitly convert btree=None -> BTreeData() in PUT

Because we next pass that btree to ._read_data() and read_data()
intentionally creates empty BTreeData() when btree is initially None.
Reviewed-by: Romain Courteaud <romain@nexedi.com>

product/ERP5/Document/BigFile.py

@@ -352,8 +352,6 @@ class BigFile(File):
         else:
 
           btree = self._baseGetData()
-          if btree is None:
-            btree = BTreeData()
 
       else:
         RESPONSE.setHeader('X-Explanation', 'Can not parse range')