Avoid usage of an empty distribution for the first build

[lsoumille]

We are using this plugin with only the newFindings thresholds. By default this plugin has the side effect to interrupt the first build as it creates a distribution with 0 vulnerability if there's no previous build. I don't see the point to have this behaviour.

The purpose of this PR is to ignore the newFindings checks for the first build in Jenkins. This has no effect if totalFindings thresholds are used.

We've tested succesfully in our Jenkins instance. If you think that this PR is appropriate I will update the changelog file.

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[sephiroth-j]

I don't see the point to have this behaviour.

The original intention was to first create a build without thresholds that represents a baseline of the current (and perhaps accepted set of vulnerabilities) that is then used as a reference for newer builds with thresholds. If you do not have a "baseline build", it is necessary to treat non-existent vulnerabilities as zero for the "new findings" feature to work. In this situation, new findings where the same as total findings.

I think your change makes sense as it allows the "new findings" feature to work in the intended way, namely only when a baseline has been set.