Allow password to be skipped or set to None in add_entry

[rrasmuseklund]

Fix: Allow skipping password or password=None in add_entry

This PR fixes an issue where skipping or passing None as the password to add_entry would raise a TypeError, due to lxml not accepting None when building XML elements.

According to the documentation:
https://libkeepass.github.io/pykeepass/pykeepass.html#PyKeePass.add_entry

password (str or None): the password of the new entry

The documentation already states that None is allowed, but the code did not support it.

What this PR does

• Skips adding the <Password> field entirely if password is None
• Matches the KeePass GUI behavior when no password is provided