Server throws exception when a custom config does not contain a specific key.

[MariusBrill]

When a custom config is used and a property is not set (e.g. self-monitoring) the server fails with a java.lang.NullPointerException. This also applies to other parts of the config, e.g. exporters.metrics.prometheus & exporters.metrics.influx.

The preferred behavior is that the server should in these cases fall back to the settings provided by the default config.