Fix empty-band-list crash in get_rate_fixed_charge_at_dt when all bands are filtered by BOOLEAN applicability #49

What

get_rate_fixed_charge_at_dt in fixedcharge.py raises a misleading RateConversionError ("Fixed price rate bands units should be COST_PER_UNIT") when rate_filter_bands returns an empty list. An empty list is a valid outcome when all bands in a FIXED_PRICE rate carry applicabilityValue=true and the scenario resolves the corresponding BOOLEAN property to False — meaning the charge simply does not apply to this customer.

The root cause is this check:

bands = ru.rate_filter_bands(rate, scenario, library)
band_rate_units = {band["rate_unit"] for band in bands}   # empty set when bands=[]
if "COST_PER_UNIT" not in band_rate_units:                # always True for empty set → raises
    raise RateConversionError(rate, "Fixed price rate bands units should be COST_PER_UNIT")

Why

This crashes URDB conversion for any utility that models opt-in charges as FIXED_PRICE rates with BOOLEAN applicability guards. BGE has two such rates ("Competitive Billing", "Smart Meter Opt-Out Charge"). A standard residential customer on utility service with no opt-out genuinely does not pay these charges — the correct contribution is $0, not an exception.

How

In get_rate_fixed_charge_at_dt, add an early return of 0.0 when rate_filter_bands returns an empty list:

def get_rate_fixed_charge_at_dt(scenario, library, rate, dt):
    bands = rate_filter_bands(rate, scenario, library)
    if not bands:
        return 0.0
    ...existing logic...

Deliverables

PR adding the early-return guard in get_rate_fixed_charge_at_dt
Unit test: construct a synthetic FIXED_PRICE rate with a single band carrying applicabilityValue=true and a scenario where the BOOLEAN property is False; assert the function returns 0.0 rather than raising