odd behaviour when registered plugins dont have a label and order

[hannesdelbeke]

a strange bug I ran in when creating a new setup a little to quickly.

when none of the registered plugins have a label and order , pyblish behaves unpredictably, without throwing any errors.

pyblish base returns an empty context on collection/validation.
pyblish lite runs the collector and validator, but then resets instantly after running. So failed nodes then go from red back to green.

after adding label, order, and optional to both the collect and validate plugin, everything started working again as expected.

since no errors were raised it took a while to figure out what was wrong.
ideally the user would always set these ofcourse, and pyblish warns the user when they are not set.