Empty qualifier is represented by an empty string, to avoid nullpointer ...

...exceptions.

Caused by: java.lang.NullPointerException
        at java.text.RuleBasedCollator.compare(RuleBasedCollator.java:351)
        at org.jenkinsci.plugins.repoclient.client.Version.compareTo(Version.java:272)
        at org.jenkinsci.plugins.repoclient.client.Version.compareTo(Version.java:41)
        at java.util.ComparableTimSort.binarySort(ComparableTimSort.java:232)
        at java.util.ComparableTimSort.sort(ComparableTimSort.java:158)
        at java.util.ComparableTimSort.sort(ComparableTimSort.java:146)
        at java.util.Arrays.sort(Arrays.java:472)
        at java.util.Collections.sort(Collections.java:155)
        at org.jenkinsci.plugins.repoclient.client.MavenRepositoryClient.sortVersions(MavenRepositoryClient.java:96)
        at org.jenkinsci.plugins.repoclient.client.MavenRepositoryClient.getVersions(MavenRepositoryClient.java:84)
        at org.jenkinsci.plugins.repoclient.RepositoryClientParameterDefinition.getChoices(RepositoryClientParameterDefinition.java:72)
        ... 155 more