cant generate p2wpkh address for Litecoin  semantic is None

```
from hdwallet.symbols import  LTC
from hdwallet import HDWallet
hdwallet: HDWallet = HDWallet(symbol=LTC)
hdwallet.from_xpublic_key(xpublic_key='zpub...', strict=False) #'p2wpkh'

```
Getting  AttributeError: 'NoneType' object has no attribute 'upper'
 , line 343, in is_root_xpublic_key
    semantic.upper().

semantic is None for LTC p2wpkh
