Return the "classification-by-ip" as protocol results only if no other
results are available.
In particular, never return something like
"protocol_by_port/protocol_by_ip" (i.e. `NTP/Apple`,
BitTorrent/GoogleCloud`, `Zoom/AWS`) because this kind of classification
is quite confusing, if not plainly wrong.
Notes:
* the information about "classification-by-ip" is always available, so
no information is lost with this change;
* in the unit tests, the previous classifications with confidence
`NDPI_CONFIDENCE_DPI_PARTIAL` were wrong, as noted in #1957
