POM: "fix" empty contributors

[Stefan Helfrich]

Some nitpicking because I looked at the POM: according to https://github.com/scijava/pom-scijava-base/blob/master/README.md#how-to-override-a-field-with-an-empty-value, pom.xml#L58-L62 should read:

<contributors>
  <contributor>
    <name>None</name>
  </contributor>
</contributors>

[Niko Ehrenfeuchter]

Thanks, @stelfrich !