The "Electricity Purchases" are incorrectly calculated when hourly buy rates of zero are specified.

To reproduce with the 2024.12.12.ssc.297 release candidate:

1. Create a PV Single Owner case
2. Select "Use retail electricity rate(s)" on the Electricity Purchases page
3. Specify a Buy rate of zero
4. Simulate
5. View the expected "Electricity purchase" O and M value to be zero
6. Change to "Use hourly (subhourly) buy rates instead of TOU rates on the Electricity Purchases page
7. Specify buy rate of 0 for all 8760 values
8. Simulate - get the unexpected non-zero "Electricity purchase" O and M value

Test file
SAM_1964.sam.zip

Expected results in test file

1. "default PV SO" and "hourly default rate" simulation results match - which they do with default buy rate of 4.5 cents/kWh specified as a buy rate and as an hourly value.
2. expect the cash flow line item for "Electricity purchase" Operating Expense value to be zero for both the "zero hourly purchases" and the "zero buy rate" which they are not zero for the "zero hourly purchases" case