Fixed incorrect interface nil check in protoc-gen-openapi (#249)
Fixed incorrect interface nil check in protoc-gen-openapi (#249)

The `annotation` variable is an interface and was only checked for `nil`, but not if `nil` would be assigned to the interface.
This lead to a crash, if a method within a service was not annotated at all.

Closes #248
google/gnostic,google/gnostic,google/gnostic