[MNG-7780] DefaultArtifact.equals throws NullPointerException if o.version is null

[CrazyHZM]

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[slachiewicz]

Please add also test case

[CrazyHZM]

Please add also test case

Done.

[gnodet]

Please simplify the logic which has not been updated since JDK 1.7

[CrazyHZM]

Please simplify the logic which has not been updated since JDK 1.7

@gnodet Done.