samdb: add prepare_attr_replace() method

samdb: add prepare_attr_replace() method

Add a method to prepare a given Message to replace the given attribute.
If the given new value is None or the old value and the new value are
the same, do nothing.
If the new value is empty, prepare to replace the given attribute with
[].
Else prepare to replace the given attribute with the new value.

Use this for samdb.modify(msg).

Signed-off-by: Jule Anger <ja@sernet.de>
Reviewed-by: Björn Baumbach <bb@samba.org>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>