Allow not specifying any plugin file

[PierreBtz]

It's currently not possible to run without specifying any plugin file.

[oleg-nenashev]

IIRC from #347 , not specifying plugins does not fully work at the moment. Bundled plugins are in classpath and largely work as extension points, but Jenkins.instance.oluginManager.getPlugin() returns null for such plugins. It does not block this pull request, but it would definitely benefit from some documentation until the metadata packaging issue is resolved

[PierreBtz]

MMhh, I must admit I went with the simplest of pipeline (echo 'hello world') and I can confirm this not very realistic use case works. I'll try to play a bit more and drop a line in the doc.

[oleg-nenashev]

@PierreBtz Should not be a problem once the new Packaging flow is finalized. I plan to inject bundled plugin metadata so that the JFR plugin manager properly loads it even if there is no HPIs referenced