CoordinateBuilder should set null for empty parts of G:A:P:C:V

CoordinateBuilder, when given "G:A:::V", should set P and C to null. Having them empty strings is no good.
Consider machine-provided lists contaning:
org.foo:foo-bar:::4.1.2
This should end with the same result as parsing
org.foo:foo-bar:4.1.2
whereas it ends up with P and C set to empty strings, and forces the user of the api to do extra check for an empty string value.

Related:
"Malformed coordinate. Should be groupId:artifactId:[packaging]:[classifier]:[version]");
should read
"Malformed coordinate. Should be groupId:artifactId:[packaging:[classifier:]][version]");

And javadoc's
<groupId>:<artifactId>[:<packaging>[:<classifier>]]:<version>
should read
<groupId>:<artifactId>[:<packaging>[:<classifier>]]:[<version>]