Title: Vnstock().stock(source=...) causes AttributeError on None symbol #191
Description
When using:
from vnstock import Vnstock
stock = Vnstock().stock(source='VCI')
It crashes with:
AttributeError: 'NoneType' object has no attribute 'upper'
Because symbol is None and passed into get_asset_type() without validation.
Please either:
Update the docs to require symbol explicitly, or
Default symbol to empty string, or
Raise ValueError("symbol must not be None") earlier for clarity