Comment 5 for bug 1838396

the error if throw in submit to mysql. from the stack, neutron server set '{}' to vnic_type.By check the code, I found in line 430(plugins/ml2/plugin.py):
vnic_type = attrs and attrs.get(portbindings.VNIC_TYPE)

if attrs = {}, vnic_type will be assigned {}, so we can encounter this bug.
I have submitted a patch. https://review.opendev.org/#/c/673486/