That reason is because inspect.getfilename cannot return anything because the arg function is a lambda, as we can see from the source code I linked:
check_info["fast_lta_volumes"] = {
# ...
"snmp_scan_function": lambda oid: (oid(".1.3.6.1.2.1.1.2.0").startswith(
".1.3.6.1.4.1.8072.3.2.10") and oid(".1.3.6.1.4.1.27417.5.1.1.2")),
}