DropwizardAppRule should not require a configPath to operate.

We use dropwizard with no configuration file (no *.yml files). So the DropwizardAppRule integration testing helper should allow the same. ImmutableMap does not accept null values, so it had to be converted to a builder and the `file` entry wrapped with a conditional.

I'm not sure if there are any other assumptions around this. So far it "just works" for us to pass `null` as the `configPath` argument with this change in the DropwizardAppRule.