Fix 'First Charge' field to accept '0' for zero charge in upgrades/downgrades

I want the 'First Charge' field in the advanced upgrade/downgrade options to correctly recognize '0' as a zero charge. Currently, entering '0' still results in a prorated charge, and I have to enter '00' to make it work correctly. This is important for ensuring customers aren't accidentally charged when we intend for a free transition.