fix(promote_chat_member): add fallback empty string rank

[MrMissx]

rank value error when called via Chat.promote_member which pass None rank as default.

Related Logs

```shell Traceback (most recent call last): File "/home/mrmiss/test.py", line 159, in debug await chat.promote_member(user_id=user.id, privileges=bot.privileges) File "/home/mrmiss/python3.9/site-packages/pyrogram/types/user_and_chats/chat.py", line 997, in promote_member return await self._client.promote_chat_member( File "/home/mrmiss/python3.9/site-packages/pyrogram/methods/chats/promote_chat_member.py", line 90, in promote_chat_member await self.invoke( File "/home/mrmiss/python3.9/site-packages/pyrogram/methods/advanced/invoke.py", line 80, in invoke r = await self.session.invoke( File "/home/mrmiss/python3.9/site-packages/pyrogram/session/session.py", line 409, in invoke return await self.send(query, timeout=timeout) File "/home/mrmiss/python3.9/site-packages/pyrogram/session/session.py", line 327, in send message = self.msg_factory(data) File "/home/mrmiss/python3.9/site-packages/pyrogram/session/internals/msg_factory.py", line 38, in __call__ len(body) File "/home/mrmiss/python3.9/site-packages/pyrogram/raw/core/tl_object.py", line 80, in __len__ return len(self.write()) File "/home/mrmiss/python3.9/site-packages/pyrogram/raw/functions/channels/edit_admin.py", line 95, in write b.write(String(self.rank)) File "/home/mrmiss/python3.9/site-packages/pyrogram/raw/core/primitives/string.py", line 32, in __new__ return super().__new__(cls, value.encode()) AttributeError: 'NoneType' object has no attribute 'encode' ```