Add NoneType check to class Entry

[langer69]

In class Entry

pykeepass/pykeepass/entry.py

Lines 27 to 30 in 7b9219b

	def __init__(self, title=None, username=None, password=None, url=None,
	notes=None, otp=None, tags=None, expires=False, expiry_time=None,
	icon=None, autotype_sequence=None, autotype_enabled=True, autotype_window=None,
	element=None, kp=None):

title, username, password accept None as input.

pykeepass/pykeepass/entry.py

Lines 42 to 46 in 7b9219b

	self._element.append(E.String(E.Key('Title'), E.Value(title or "")))
	self._element.append(E.String(E.Key('UserName'), E.Value(username or "")))
	self._element.append(
	E.String(E.Key('Password'), E.Value(password or "", Protected="True"))
	)

But E.Value() reject None.
Since keepass allows entry with no title username or password,this will cause exception.

if title:
    self._element.append(E.String(E.Key('Title'), E.Value(title)))
if username:
    self._element.append(E.String(E.Key('UserName'), E.Value(username)))
if password:
    self._element.append(E.String(E.Key('Password'), E.Value(password, Protected="True")))

I think add NoneType check would be better.