Both get_balance_allowance and update_balance_allowance in client.py declare
params: BalanceAllowanceParams = None but immediately access params.signature_type
without a None check.

Calling either method without params raises an opaque error:

AttributeError: 'NoneType' object has no attribute 'signature_type'

This was also flagged during code review on #224 (async client PR) where the reviewer
noted the same bug exists in the sync client.

Suggested fix: add a guard that raises a clear ValueError before accessing params attributes.