Allow xmlfile.write() to accept a None

[Charlie_X]

This is a convenience request that xmlfile.write(None) should not raise an exception as it currently does. I find it fairly common to have to want to pass elements coming from functions or methods that might return an Element or None. This means wrapping the call to xmlfile.write() in conditional code whereas "do nothing" is required. I don't know if this is a common use case but I can imagine it might be.

[scoder (scoder)]

Seems reasonable and won't hurt.
https://github.com/lxml/lxml/commit/904c9742b23914ce772a4c10013855b0996d5f73