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

apps/protoc-gen-openapi/generator/openapi-v3.go

diff --git a/apps/protoc-gen-openapi/generator/openapi-v3.go b/apps/protoc-gen-openapi/generator/openapi-v3.go
index f022312..4211c97 100644
--- a/apps/protoc-gen-openapi/generator/openapi-v3.go
+++ b/apps/protoc-gen-openapi/generator/openapi-v3.go
@@ -127,11 +127,12 @@
 			inputMessage := method.Input
 			outputMessage := method.Output
 			operationID := service.GoName + "_" + method.GoName
-			extension := proto.GetExtension(method.Desc.Options(), annotations.E_Http)
+			xt := annotations.E_Http
+			extension := proto.GetExtension(method.Desc.Options(), xt)
 			var path string
 			var methodName string
 			var body string
-			if extension != nil {
+			if extension != nil && extension != xt.InterfaceOf(xt.Zero()) {
 				rule := extension.(*annotations.HttpRule)
 				body = rule.Body
 				switch pattern := rule.Pattern.(type) {