allow optional component configurations

the maven-plugin-plugin needs to have access to the distribution repository for the project, which may or may not exist. This should be a resolvable expression: $

but a valid result of evaluating this is 'null' meaning that the mojo will work either with or without this configuration.

We need to allow optional configuration elements, so that if the evaluation result for the configuration element is null, it simply skips that configuration instead of throwing an exception.