gapic-generator-cloud docker invocation fails if --ruby-cloud-gem-name option is not supplied

when invoking gapic-generator-cloud docker like this:

```sh
$ docker run --rm --user $UID \
--mount type=bind,source=`pwd`/google/showcase/v1beta1,destination=/in/google/showcase/v1beta1,readonly \
--mount type=bind,source=`pwd`/showcase-ruby,destination=/out \
gcr.io/gapic-images/gapic-generator-ruby:latest
```
the generator will fail with the Nil reference exception.

```
/usr/local/bundle/gems/gapic-generator-0.0.1/templates/default/helpers/presenters/gem_presenter.rb:62 \
:in `namespace': undefined method `split' for nil:NilClass (ActionView::Template::Error)
```

This can be remedied by supplying the docker generator with the --ruby-cloud-gem-name parameter:
```sh
$ docker run --rm --user $UID \
--mount type=bind,source=`pwd`/google/showcase/v1beta1,destination=/in/google/showcase/v1beta1,readonly \
--mount type=bind,source=`pwd`/showcase-ruby,destination=/out \
gcr.io/gapic-images/gapic-generator-ruby:latest --ruby-cloud-gem-name=showcase
```
(this generates correctly)

There should be a sensible default for the gemname we can add to the gapic-generator-ruby to avoid this issue.