Do not allow PIFR to write a result with environment = 0 to the pifr_result database table

See #1848806: Modify pifr_server_test_get_since (or downstream functions) to screen out invalid test results with environment = 0 for potential implications.

I haven't checked the code to see if this is a possibility (i.e. do we write placeholder values with environment_id = 0, for example?), but explictly checking for invalid combinations before writing to pift_result would help avoid a number of difficult to resolve testbot issues, such as #1848714: testbot not reporting results back to issue queues, no Re-test link.