Do not use textual 'null' for dependencies without group (#725)

[Vampire]

Fixes #725

[ben-manes]

Thanks! Can you fix the test failures? It might just be some minor updates, e.g.

DependencyUpdatesSpec > Single project with flatDir repository FAILED
    Condition not satisfied:

    unresolved.collect { it.selector }.collectEntries { dependency -> [['group': dependency.group, 'name': dependency.name]: dependency.version] } == [['group': 'null', 'name': 'guava-18.0']: NONE_VERSION]
    |          |                       |                                                                                                           |                                            |
    |          [:guava-18.0:none]      [[group:, name:guava-18.0]:none]                                                                            false                                        none
    [:guava-18.0:none]
        at com.github.benmanes.gradle.versions.DependencyUpdatesSpec.Single project with flatDir repository_closure21(DependencyUpdatesSpec.groovy:359)
        at groovy.lang.Closure.call(Closure.java:412)
        at spock.lang.Specification.with(Specification.java:191)
        at com.github.benmanes.gradle.versions.DependencyUpdatesSpec.Single project with flatDir repository(DependencyUpdatesSpec.groovy:358)

[Vampire]

Hm, I'm not sure about the undeclared failure, whether my changes just surfaced that there is some other bug in Resolver.kt or whether my changes are problematic.
With the current state the guice dependency is also treated as hidden.

[Vampire]

Can you maybe have a look?

[Vampire]

Actually if I comment out the if (originalCoordinate == null && resolvedCoordinate.groupId != "none") { and just keep the else, the tests in DependencyUpdatesSpec all pass.
Maybe it is not necessary and not appropriate actually as there is already Ignore undeclared (hidden) dependencies at an earlier place in that file?

[Vampire]

I pushed a version with that change, let's see whether something else breaks or you say this actually is necessary. :-)

[Vampire]

Well, the tests passed with that change :-D

[ben-manes]

It looks good to me. Running it against Caffeine and I don't see any of the hidden dependencies in the report (bundled ant, jacoco) and the undeclared category from v39 has unit tests.