Add support for Mono.fromRunnable(Runnable): Mono<T> generic inference

[gregturn]

Add additional inference to support returning a Mono.empty based on return type.

That way this...

@Override
public Mono<MongoSession> findById(String id) {

	return findSession(id)
		.map(document -> convertToSession(this.mongoSessionConverter, document))
		.filter(mongoSession -> !mongoSession.isExpired())
		.switchIfEmpty(Mono.defer(() -> delete(id).then(Mono.empty())));
}

can be come this...

@Override
public Mono<MongoSession> findById(String id) {

	return findSession(id)
		.map(document -> convertToSession(this.mongoSessionConverter, document))
		.filter(mongoSession -> !mongoSession.isExpired())
		.switchIfEmpty(Mono.defer(() -> delete(id).then());
}