Envoy emits a bogus "failed to disable extension" message at startup

*Title*: Envoy emits a bogus "failed to disable extension" message at startup

*Description*:

```
[jpeach@sk1 envoy]$ ./bazel-bin/external/envoy/source/exe/envoy-static
[2019-12-17 15:00:42.867][25530][warning][misc] [external/envoy/source/server/options_impl.cc:343] failed to disable invalid extension name ''
...
```

*Relevant Links:* #9125 