Fix NPE caused by empty Group-Id entry in MANIFEST.MF

Ancient plugins (such as locks-and-latches-plugin) do not have this fields set. Use "org.jvnet.hudson.plugins" as a default.

```
java.lang.NullPointerException
	at org.jvnet.hudson.update_center.HPI.isAuthenticJenkinsArtifact(HPI.java:235)
	at org.jvnet.hudson.update_center.PluginHistory.findYoungestJenkinsArtifact(PluginHistory.java:102)
	at org.jvnet.hudson.update_center.PluginHistory.addArtifact(PluginHistory.java:86)
	at org.jvnet.hudson.update_center.LocalDirectoryRepository.listHudsonPlugins(LocalDirectoryRepository.java:143)
	at org.jvnet.hudson.update_center.Main.buildPlugins(Main.java:269)
	at org.jvnet.hudson.update_center.Main.buildUpdateCenterJson(Main.java:199)
	at org.jvnet.hudson.update_center.Main.run(Main.java:163)
	at org.jvnet.hudson.update_center.Main.run(Main.java:140)
	at org.jvnet.hudson.update_center.Main.main(Main.java:127)
```