Allow feature value matchers to be optionally composed

[markhobson]

A common use-case is to compose feature matchers only when their values are specified. The current API makes this hard.

Consider a simple example:

compose("a person with", hasFeatureValue("title", Person::getTitle, expected.getTitle()))
		.and(hasFeatureValue("first name", Person::getFirstName, expected.getFirstName()))
		.and(hasFeatureValue("last name", Person::getLastName, expected.getLastName()));

If any of the expected values are null then a matcher is still created and added to the composed matcher. Perhaps, if the value was an Optional then hasFeatureValue could return an Optional<Matcher> which and could then only append if not empty.