Default implementation for LinkResolverFactory

[DaveJarvis]

https://github.com/vsch/flexmark-java/blob/c0313d67e0146292a10d04eb8944faff991579e9/flexmark/src/main/java/com/vladsch/flexmark/html/LinkResolverFactory.java#L13

The methods getAfterDependents and getBeforeDependents return null in a number of cases. Rather than forcing a null implementation detail upon implementations, perhaps a default implementation would be worthwhile?

Having such default method implementations would potentially eliminate the need for IndependentLinkResolverFactory.